An ultra-high-resolution small-angle neutron scattering (USANS) doublecrystal diffractometer (DCD) is now in operation at the NIST Center for Neutron Research (NCNR). The instrument uses multiple reflections from large silicon (220) perfect single crystals, before and after the sample, to produce both high beam intensity and a low instrument background suitable for small-angle scattering measurements. The minimum detector background to beam intensity ratio (noise-to-signal, N/S) for q ! 5 Â 10 À4 Å À1 is 4 Â 10 À7 . The instrument uses 2.38 Å wavelength neutrons on a dedicated thermal neutron beam port, producing a peak flux on the sample of 17 300 cm À2 s À1 . The typical measurement range of the instrument extends from 3 Â 10 À5 Å À1 to 5 Â 10 À3 Å À1 in scattering wavevector (q), providing information on material structure over the size range from 0.1 mm to 20 mm. This paper describes the design and characteristics of the instrument, the mode of operation, and presents data that demonstrate the instrument's performance.

Conventional (e.g. MgH 2 ) and complex hydrides (e.g. alanates, borohydrides, and amides) are the two primary classes of solid-state hydrogen-storage materials. [1][2][3] Many of these "high-density" hydrides have the potential to store large amounts of hydrogen by weight (up to 18.5 wt % for LiBH 4 ) and/or volume (up to 112 g L À1 for MgH 2 ), values that are comparable to the hydrogen content of gasoline (15.8 wt %, 112 g L À1 ). However, all known hydrides are inadequate for mobile storage applications due to one or more of the following limitations: a) unfavorable thermodynamics (they require high temperatures to release hydrogen [4] ), b) poor kinetics (low rates of hydrogen release and uptake), c) decomposition pathways involving the release of undesirable by-products (e.g. ammonia), and/or d) an inability to reabsorb hydrogen at modest temperatures and pressures (i.e. "irreversibility").In spite of these drawbacks, renewed interest in complex hydrides has been stimulated recently by substantial improvements in their kinetics and reversibility [5,6] provided by catalytic doping (e.g. TiCl 3 -doped NaAlH 4 ), [7,8] and by thermodynamic enhancements achieved through reactive binary mixtures [9] such as LiNH 2 /MgH 2 , [10,11] LiBH 4 /MgH 2 , [12] and LiNH 2 /LiBH 4 . [13,14] These compositions, previously termed "reactive hydride composites", [15] represent the state-of-the-art in hydrogen-storage materials; compared to their constituent compounds, they exhibit improved thermodynamic properties, higher hydrogen purity, and, in some cases, reversibility. The desorption behavior of these previously studied composites is illustrated in Figure 1 a. It is evident from the hydrogen desorption profile (top panel) that the composites generally desorb hydrogen at significantly lower temperatures than their individual components. For example, the lowest temperature reaction, which involves a Figure 1. a) Hydrogen (top) and ammonia (bottom) kinetic desorption data as a function of temperature (5 8C min À1 to 550 8C) for the ternary composition (blue trace) and its unary and binary constituents. Hydrogen desorption is measured in weight percent (wt %) to 1 bar whereas relative ammonia release is measured as partial pressure (torr) in a flow-through set-up (100 sccm Ar). b) Ternary phase space defined by unary compounds (nodes), LiBH 4 (pink), MgH 2 (purple), and LiNH 2 (orange) and the binary mixtures (edges), LiBH 4 /MgH 2 (gray), MgH 2 / LiNH 2 (green), and LiNH 2 /LiBH 4 (red). The present ternary composition, which is a 2:1:1 mixture of LiNH 2 , LiBH 4 , and MgH 2 , and previously investigated binaries, are identified.

The structure and dynamics of poly(ethylene oxide) (PEO) intercalated in the galleries of a fluoromica inorganic clay were studied by spin-label electron spin resonance (ESR), XRD, FTIR, and DSC. The polymer was end-labeled by attachment of a nitroxide radical. Basic structural information was determined by XRD, while DSC and FTIR revealed that the crystallization of the PEO was inhibited in the clay galleries. The temperature variation of the ESR spectra from the spin-label was simulated based on the “macroscopic order with microscopic disorder” (MOMD) model. For PEO intercalated in the narrow (0.33 nm) clay galleries, the ESR spectra indicated a very low segmental mobility even at high temperature, 410 K, which was attributed to the strong polymer interaction with the charged mica platelets. In wider (0.83 nm) galleries, however, the parameters used to simulate the ESR spectra of the nitroxide labels reflected a lowering of the PEO segmental density: In this sample, the ESR spectrum consisted of two distinct contributions from slow- and fast-motional components, and the relative intensity of the fast component increased with an increase in temperature. The two spectral components were attributed to segments located close to, and away from, the polar solid walls in the gallery, respectively. Interestingly, the fast-motional component had higher mobility compared to that of PEO chains adsorbed on the fluoromica surface. In addition, the activation energy of the segmental motion in the fast-motional component was lower compared to that of bulk PEO. The low segmental density and reduced cooperative motion with neighboring segments are considered the main factors leading to the fast PEO chain motion with low activation energy.

Bonse-Hart double-crystal diffractometers (DCD) with multibounce channel-cut crystals show rocking curves which depart dramatically in their wings from dynamical diffraction theory. This intrinsic background is many orders of magnitude higher than the predictions of dynamical diffraction theory. This effect has been studied at the ultra-small-angle neutron scattering (USANS) facility at Oak Ridge National Laboratory by the measurement of rocking curves from different volume elements of a thick single-bounce Si(lll) perfect crystal and from triple-bounce channel-cut Si(lll) crystals. Analysis of these data, together with the rocking curves from an X-ray Bonse-Hart DCD, allows it to be established that the rocking curve of the multibounce channel-cut crystals contains parasitic scattering generated at the surface of the crystals. The intensity of this component can be reduced by very deep etching of the crystal surface. Using this technique, the signal-to-noise ratio of the Bonse-Hart DCD at the ORNL USANS facility has been improved by one order of magnitude to ~,5 x 105 [I(0)/I(0 = 10")].