The discovery of superconductivity at 39 K in MgB 2 1 raises many issues. One of the central questions is whether this new superconductor resembles a hightemperature-cuprate superconductor or a lowtemperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks 2 . Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive 3,4 . These studies of MgB 2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains 3 . Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determine d by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.The principal samples were synthesized by direct reaction of bright Mg flakes (Aldrich Chemical) and sub-micron amorphous B powder (Callery Chemical). Starting materials were lightly mixed in half-gram batches, and pressed into pellets. These pellets were placed on Ta foil, which was in turn placed on Al 2 O 3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H 2 for 1 hour at 600 C, 1 hour at 800 C, and 1 hour at 900 C, and then lightly ground. The resulting powders were pressed into pellets and then hot pressed at 10 kbar at temperatures between 650 and 800 °C for periods between 1 and 5.5 hours. Disks ~4 mm in diameter and ~1 mm thick were cut from these pellets for property characterization. As noted later, this process cannot yet be considered optimum.Magnetization properties were examined in SQUID and vibrating sample magnetometers (VSM) in applied fields up to 14 T from 4.2 to above T c . Figure 1 shows onset T c values of 37-38 K for the above samples and for commercial MgB 2 powder (99.5%, ~2 µm diameter by examination, CERAC). Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in su...

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB2. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E2g symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B σ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to quantitatively explaining the observed high Tc and boron isotope effect in MgB2 PACS numbers: 63.20. Ry, 63.20.Kr, 74.25.Jb, 74.25.Kc The recent discovery of superconductivity at 40 K in the MgB 2 binary alloy system [1] has triggered enormous interest in the structural and electronic properties of this class of materials. The system has a very simple crystal structure [2], where the boron atoms form graphitelike sheets separated by hexagonal layers of Mg atoms (see inset to Fig. 1). Our pseudopotential plane wave band structure calculations show that the bands near the Fermi level arise mainly from the p x,y σ bonding orbitals of boron, while the Mg does not contribute appreciably to the conductivity, in good agreement with initial reports from other groups [3][4][5]. In the case of graphite these σ bands are full, but for MgB 2 they are partially unoccupied, creating a hole-type [6] conduction band like the high-T c cuprates. In contrast to the cuprates, however, the normal-state conductivity is three-dimensional in nature instead of being highly anisotropic, thus eliminating the "weak-link" problem that has plagued widespread commercialization of the cuprates. The normal-state conductivity [6][7][8] is also one to two orders-of-magnitude higher than either the Nbbased alloys or Bi-based cuprates used in present day wires, and this feature combined with low cost and easy fabrication [8,9] could make this class of materials quite attractive for applications.From a fundamental point of view the central question is whether the high T c in this new system can be understood within the framework of a conventional electronphonon mechanism, or a more exotic mechanism is responsible for the superconducting pairing. The observed boron isotope effect [10] argues for an electron-phonon mechanism, while the positive Hall coefficient [6] suggests similarities with the cuprates [11]. To answer this question, we have carried out inelastic neutron scattering measurements of the phonon density of states, and compare these results with detailed first-principles calculations of the lattice dynamics (and electronic) calculations for MgB 2 . Excellent agreement is found between theory and experiment. More importantly, the numerical results demonstrate that the in-plane boron phonons near the zone-center (with E 2g symmetry at Γ) are very anharmonic and strongly coupled to the partially occupied c...

A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs 2.5 H 0.5 PW 12 O 40 , has been attempted by combining the new results obtained with solid-state NMR, XRD, SEM, and N 2 porosimetry with the data reported previously. The precipitates of Cs 2.5 H 0.5 PW 12 O 40 just formed from aqueous solutions of H 3 -PW 12 O 40 and Cs 2 CO 3 consist of ultrafine crystallites in which the acid form, H 3 PW 12 O 40 , is epitaxially deposited on the surface of Cs 3 PW 12 O 40 crystallites. Calcination of the precipitates brings about the migration of H + and Cs + in the solid to form a nearly uniform solid solution in which protons distribute randomly through the entire bulk, as revealed by XRD and 31 P solid-state NMR. Impregnation of Cs 3 PW 12 O 40 with the aqueous solution of H 3 PW 12 O 40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the analysis of N 2 desorption isotherm showed that Cs 2.5 H 0.5 PW 12 O 40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH 3 sorption indicated the presence of very strong acid sites on Cs 2.5 H 0.5 PW 12 O 40 . High catalytic activity of Cs 2.5 H 0.5 -PW 12 O 40 reported for solid-liquid reaction systems is thus principally attributed to the strength and number of acid sites and the mesoporous structure appropriate for the rapid diffusion of molecules.

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The states and dynamic behavior of acidic protons and water molecules in solid H3PW12O40·nH2O (0 < n < 6), which would be closely related to its pseudoliquid phase catalysis, were quantitatively elucidated by the comprehensive application of 31P, 1H, and 17O magic-angle spinning (MAS) NMR. 1H and 17O MAS NMR were sensitive to the local environment in the pseudoliquid phase (e.g., hydrogen bonding), and 31P MAS NMR was effective especially to quantify the states of the protons. At 173 K, several peaks appeared in the 31P MAS NMR spectra that were reasonably assigned to phosphorus atoms in polyanions (PW12O40 3-) having different numbers of acidic proton(s) directly attached to them. Hence, the acidic protons reside either directly on the polyanions as “isolated acidic protons” or as H3O+ or H5O2 +, and the amounts of these species were determined as a function of water content. The relative intensity of the 31P MAS NMR peaks obeyed binomial distribution for all the range of 0 < n < 6, which shows that the isolated acidic protons and protonated water molecules (H3O+ and H5O2 +) are distributed uniformly (i.e., randomly) in the solid. The distribution of these species was well explained by the random removal of water from the solid bulk. At 298 K, the 31P MAS NMR peaks coalesced, revealing that the isolated acidic protons, which are the origin of the strong acidity in the pseudoliquid phase, migrate between the neighboring polyanions much faster than catalytic reactions. This is the first quantitative observation of the protons in hydrated heteropolyacids by spectroscopic methods.

Since the discovery of mesoporous silica, [1][2][3] new organicinorganic nanocomposites based on mesoporous silica materials have been extensively investigated in the development of functional materials in various fields.[4] The grafting of organic groups onto the pore walls of the silica [5] has provided novel materials for catalysis, [6] heavy-metal ion adsorption, [7] photocontrollable molecular storage, [8] gas separation, [9] and molecular recognition. [10][11][12] Since the chemical functionalities of these materials have been ascribed mainly to the organic moiety, a promising strategy toward new functions is to design an inorganic-organic cooperative mechanism in nanostructured materials. [11,12] Solid acid catalysts have served as important functional materials in about 180 industrial processes in the petroleum refinery industry and in the production of chemicals.[13] In contrast, a significant number of acid-catalyzed reactions, such as Friedel-Crafts reactions, esterification, and hydrations, are still carried out by using conventional acids, such as H 2 SO 4 and AlCl 3 . In particular, for the reactions in which water participates as a reactant or product, such as hydrolysis, hydration, and esterification, only a few solid acids show acceptable performances. [14][15][16] The development of new water-tolerant solid acids is expected to have a major impact in industrial applications as well as in scientific aspects. One of the major difficulties concerned with the use of solid acids is the severe deactivation of the acid sites by water, and in fact, most solid acids lose their catalytic activity in aqueous solutions.We have overcome this difficulty by designing acid catalysts comprising polyoxometalate (hetero-polyacid) molecules and organografted mesoporous silica. We found that the acidic protons in the hydrophobic environment of organomodified mesoporous silica show extremely high catalytic activity for ester hydrolysis in water. Figure 1 illustrates the concept of the nanostructured catalyst. Two kinds of organic groups, n-octyl and 3-aminopropyl, were grafted onto the pore walls of mesoporous silica SBA-15. [3] The aminopropyl groups immobilize the H 3 PW 12 O 40 polyoxometalate anions on the pore walls, while the octyl groups (ca. 1 nm in length) form hydrophobic regions around the polyanions. It was found that water and reactant molecules can penetrate into the nanospaces through the remaining spaces at the centers of the SBA-15 pores. In the preparation of the catalyst, first alkyl groups and then 3-aminopropyl groups (AP groups) were grafted on SBA-15 (pore diameter 7.5 nm, BET surface area 458 m 2 g À1 ) to obtain organomodified materials, denoted by C n -AP-SBA, where n is the number of carbon atoms of the alkyl group. After neutralization of the amino groups with hydrochloric acid, Figure 1. Schematic illustration outlining the preparation and structure of the catalysts. Octyl and 3-aminopropyl groups were subsequently grafted on the pore walls of mesoporous silica SBA-15, followed by immobil...