The analysis of small-and ultra-small-angle neutron scattering data for sedimentary rocks shows that the pore-rock fabric interface is a surface fractal ͑D s 2.82͒ over 3 orders of magnitude of the length scale and 10 orders of magnitude in intensity. The fractal dimension and scatterer size obtained from scanning electron microscopy image processing are consistent with neutron scattering data.[S0031-9007(99)08945-0] PACS numbers: 61.43.Hv, 61.12.Ex, Owing to the limited size range over which the fractal properties are usually observed, the issue of the apparent fractal geometry of various natural objects is a contentious one. In their critique of 96 recent reports on the fractality of a wide range of physical systems, Avnir et al. pointed out the contradiction between the narrow range of the appropriate scaling properties for declared fractal objects (centered around 1.3 orders of magnitude) and the public image of the status of experimental fractals [1], which for rocks has previously been based on limited experimental evidence (about 1.5 decades in length scale). A notable exception is the x-ray study of Bale and Schmidt on coals (Ref. [2], 2 decades in length scale, 7.5 decades in intensity). In this study we extend the range of length scales studied for rocks to over 3 decades (10 decades in intensity) and show that sedimentary rocks are in fact one of the most extensive fractal systems found in nature.Sedimentary rocks are formed from a mixture of organic and inorganic debris deposited in an aqueous environment, buried and compacted at elevated temperatures over geological periods of time. Remarkably, there is no percolation threshold observed in sedimentary rocks, which indicates a microstructure more complex than one originating from just a collection of compacted grains. According to the antisintering hypothesis of Cohen, the rock/pore interface evolves by maximizing the internal surface area in response to the secular equilibrium between the rock matrix and the formation brine [3]. Various studies performed on rocks of different origin and lithology over length scales in the range 20 Å to 100 mm have shown that sedimentary rocks are often effective fractals [4]. Experimental tools used in these studies include molecular adsorption [5], microscopic techniques [6,7], and small-angle scattering (SAS) methods. SAS methods are particularly well suited for testing the porematrix interface: They are noninvasive, average over the entire sample volume, and include correlation information. Previous small-angle neutron and x-ray scattering (SANS and SAXS) studies on shales [8-10] and sand-stones [7,9,11] demonstrated the surface fractal geometry of the pore-matrix interface in the scale range 20 Å to about 2000 Å.Recent progress in neutron scattering instrumentation enables one to access the microstructure of rocks well beyond the conventional SANS Q limit of Q min 3 3 10 23 Å 21 . The 80-m SANS instrument D11 at ILL [12] has resolution Q min 8 3 10 24 Å 21 and the Bonse-Hart geometry USANS facility at ORNL [13] c...

No abstract

The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10 nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10 nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron timeof-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The α-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude fQ(P ) reveals the onset of β relaxation. The β-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure Pc(T ). α and β relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Γ ∝ nT −1/4 . 64.70. Pf, 62.50.+p, 61.25.Em, The glass transition can be induced by decreasing the temperature or by increasing the pressure. For practical reasons, most experimental investigations concentrate on temperature effects at ambient pressure P 0 = 0.1 MPa, although there is a clear interest in studying supercooled liquid dynamics in the full P, T parameter space. Variable pressure measurements have been decisive in demonstrating that density is not the only driving force behind the glass transition [1][2][3]. In the microscopic approach of mode-coupling theory [4], both pressure and temperature control the dynamics through variations of the static structure factor. Close to the dynamic cross-over, the effect of these variations can be expressed through a single separation parameter which however is not given by density alone, as was confirmed by depolarised light scattering in the fragile liquid cumene [3].Here we report on incoherent neutron scattering in the fragile van der Waals liquid ortho-terphenyl (OTP, T g (P 0 ) = 243 K) at variable pressure. As function of temperature, the microscopic dynamics of OTP has been studied extensively by neutron scattering [5][6][7] [5,10] and from direct observation of fast β relaxation [6-9] a cross-over was located at T c (P 0 ) = 290 ± 5K [16].OTP (Aldrich) was purified by repeated crystallisation from liquid methanol and vacuum destillation. An Al7049.T6 high pressure cell of the Institut LaueLangevin (ILL) with a ratio of outer to inner diameter of 2 allowed us to attain up to 250 MPa at temperatures up to 335 K; helium was used to transmit the pressure.The experiments were performed using backscattering (IN16) and time-of-flight (IN6) spectrometers of the ILL. Incident wavelengths 6.27Å −1 and 5.12Å −1 gave resolutions (fwhm) of about 1 µeV and 80 µeV, respectively.Vanadium was used to calibrate the detectors and to yield the resolution functions. The measured transmission of the empty pressure cell at P 0 was about 84%, that of the sample about 88%. Self shielding and multiple scattering turned out be negligible at large scattering angles. This was demonstrated by comparison of spectra taken with and without the pressure cell at 290 K: the normalised intermediate ...

In order to clarify the mechanisms of hydrogen diffusion in the cubic (C15) and hexagonal (C14) modifications of Laves phase ZrCr2, we have performed high-resolution quasielastic neutron scattering measurements on C15-ZrCr2H0.45 and C14-ZrCr2H0.5 over the temperature range 10-340 K. It is found that in both systems the diffusive motion of hydrogen can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial (ZrCr2) sites and the slower hopping from one hexagon to the other. The experimental results are analysed to determine the hydrogen hopping rates and the tracer diffusion coefficients as functions of temperature. The motional parameters of hydrogen in the C15- and C14-type samples are found to be close to each other. Comparison of the motional parameters of hydrogen in ZrCr2Hx and in the related C15-type TaV2Hx shows that the localized H motion in ZrCr2 is slower, whereas the long-range H diffusion is much faster than in TaV2. These features are consistent with the difference between intersite distances in the hydrogen sublattices of ZrCr2Hx and TaV2Hx.

No abstract

No abstract

Abstract. -Structural α relaxation in liquid orthoterphenyl is studied by means of coherent neutron time-of-flight and backscattering spectroscopy over a large temperature range.Not only amplitude and relaxation time but also the spectral line shape show a significant variation with wavenumber. As expected from mode coupling theory, these variations are correlated with the static structure factor. Even far above the melting point, α relaxation remains non-exponential.The most outstanding features of structural α relaxation in glass-forming liquids are a strong temperature dependence of the relaxation time and the stretching phenomenon of the response function [1]. Very often, the correlation function Φ X (t) of some observable X obeys time temperature superposition (TTS): by a shift of the characteristic relaxation time τ X (T ) the response functions at different temperatures can be superimposed Φ X (t; T ) =Φ X (t), witĥ t = t/τ X (T ). While the master functionΦ X (t) varies weakly with temperature, the scale τ X (T ) increases rapidly upon cooling. The other remarkable feature is the stretching phenomenon: the decay of Φ X extends over a wider time range than expected for exponential relaxation; empirically, it is generally well described by the Kohlrausch function (KWW)Different observables X may have not only different relaxation times τ X but also different stretching exponents β X . In light scattering, for instance, β can be changed by just switching between polarized and depolarized geometry. Uncountable experiments have confirmed both observations for times varying from nanoseconds to hours, days, or longer.A microscopic explanation of stretched relaxation has been given by mode coupling theory (MCT) [2]. Within MCT, time correlations decay in a two-step process which becomes