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[en] Dehydroxylation of ettringite in an atmosphere of constant partial water vapour pressure (30-400 Torr) and controlled temperature (55-95 deg. C) yields an X-ray amorphous product containing 11-13 H₂O per ettringite formula unit. The X-ray amorphous product does, however, give electron diffraction patterns similar to those of ettringite but with a considerably reduced from ∼1.123 nm (ettringite) to 0.85 nm in the partially dehydroxylated product, termed metaettringite. The structure of metaettringite is closely related to that of fleischerite, Pb₃Ge[(OH)₆](SO₄)₂·3H₂O and the isostructural despujolsite, Ca₃Mn^IV(SO₄)₂(OH)₆·3H₂O. These minerals contain columnar units. The columnar structure is like that of ettringite but with closer packing of columns in the a direction, resembling metaettringite. The mechanism of collapse of ettringite to metaettringite, involving loss of water and motion of columns, cannot be achieved without scissoring and possibly rotation of individual columns; this and other defect-producing mechanisms result in loss of crystallinity of the metaettringite product

[en] This is a Phase I report on a project to significantly enhance existing subsurface simulation software using leadership-class computing resources, allowing researchers to solve problems with greater speed and accuracy. Subsurface computer simulation is used for monitoring the behavior of contaminants around nuclear waste disposal and storage areas, groundwater flow, environmental remediation, carbon sequestration, methane hydrate production, and geothermal energy reservoir analysis. The Phase I project was a collaborative effort between Thunderhead Engineering (project lead and developers of a commercial pre- and post-processor for the TOUGH2 simulator) and Lawrence Berkeley National Laboratory (developers of the TOUGH2 simulator for subsurface flow). The Phase I project successfully identified the technical approaches to be implemented in Phase II.

[en] Matrix diffusion is an important mechanism for solute transport in fractured rock. We recently conducted a literature survey on the effective matrix diffusion coefficient, D_m^e, a key parameter for describing matrix diffusion processes at the field scale. Forty field tracer tests at 15 fractured geologic sites were surveyed and selected for the study, based on data availability and quality. Field-scale D_m^e values were calculated, either directly using data reported in the literature or by reanalyzing the corresponding field tracer tests. Surveyed data indicate that the effective-matrix-diffusion-coefficient factor F_D (defined as the ratio of D_m^e to the lab-scale matrix diffusion coefficient [D_m] of the same tracer) is generally larger than one, indicating that the effective matrix diffusion coefficient in the field is comparatively larger than the matrix diffusion coefficient at the rock-core scale. This larger value can be attributed to the many mass-transfer processes at different scales in naturally heterogeneous, fractured rock systems. Furthermore, we observed a moderate trend toward systematic increase in the F_D value with observation scale, indicating that the effective matrix diffusion coefficient is likely to be statistically scale dependent. The F_D value ranges from 1 to 10,000 for observation scales from 5 to 2,000 m. At a given scale, the F_D value varies by two orders of magnitude, reflecting the influence of differing degrees of fractured rock heterogeneity at different sites. In addition, the surveyed data indicate that field-scale longitudinal dispersivity generally increases with observation scale, which is consistent with previous studies. The scale-dependent field-scale matrix diffusion coefficient (and dispersivity) may have significant implications for assessing long-term, large-scale radionuclide and contaminant transport events in fractured rock, both for nuclear waste disposal and contaminant remediation

[en] A study of the abrasion wear resistance of the 2.7C-17Cr-0.7Mo white cast irons with different structures alloyed with boron ranging from 0.1% to 1.3% is carried out. Eleven heat treatments were used to find the optimum treatment. Three conditions (as-cast, martensitic and austenitic) are adopted for various tests. The microstructure and three-dimensional morphology of compounds are examined by optical microscope and SEM respectively. X-ray diffractometer is employed to analyze the compound phases. A high-stress abrasive wear tests is performed with loose SiO/sub 2/ and SiC abrasives in a metal track wear tester. Another abrasive wear test is conducted with wet SiO/sub 2/ abrasives in a rubber wheel tester. The hardness and fracture toughness of these alloys was also measured. With increasing boron content fracture toughness decreases. It is noted that if the irons contained about same compound volume, the abrasion wear resistance in present wear systems are much better than the irons without boron against SiO/sub 2/ abrasives, and the toughness is equivalent to 15 Cr irons without boron. Finally, considering the wear resistance and fracture toughness, the test results would provide a basis for optimizing these properties in selecting materials for a given wear component

[en] The effects of particle size on the structure and Raman spectra of lead lanthanum titanate nanocrystalline particles are investigated. Samples with different particle sizes were prepared by the sol - gel process using different calcination temperatures. The sizes of the crystallites of particles were determined by x-ray diffraction with the aid of Scherrer's equation. Raman vibration modes of samples with various particle sizes were also studied by Raman scattering at room temperature. The results show that the tetragonal distortion of the unit cell apparently decreases with decreasing particle size: the soft A₁(1TO) phonon mode which is related to the spontaneous polarization of ferroelectric materials shifts toward lower frequency, and other peaks exhibit broadening and intensity changes with decreasing particle size. These are attributed to a phase transformation from the ferroelectric tetragonal to the paraelectric cubic phase. The above phenomena are related to a grain size-induced structural phase transition. [copyright] 2001 American Institute of Physics

[en] We calculate the collective excitation spectra of σ, ω mesons in nuclear matter by calculating the relativistic particle-hole excitation polarization insertion of the propagator in the Walecka model. In the mean field approximation and without σ-ω mixing the spectra splits into the meson branch and acoustic sound branch; for the most part the acoustic sound modes are damped. The transverse ω meson has meson branch excitation spectra only. When σ-ω mixing sets in, the acoustic sound excitation spectra of the σ meson disappears and that of the longitudinal ω meson is suppressed. In the relativistic Hartree approximation and with σ-ω mixing, all acoustic sound excitation spectra disappear and the spectra in the meson branch do not change very much for all cases. (author)

[en] A series of Monte Carlo simulations have been conducted, making use of the EJ-426 neutron scintillator detector, to investigate the potential of using hexagonal uniformly redundant arrays (HURAs) for scintillator based coded aperture neutron imaging. This type of scintillator material has a low sensitivity to gamma rays, therefore, is of particular use in a system with a source that emits both neutrons and gamma rays. The simulations used an AmBe source, neutron images have been produced using different coded-aperture materials (boron- 10, cadmium-113 and gadolinium-157) and location error has also been estimated. In each case the neutron image clearly shows the location of the source with a relatively small location error. Neutron images with high resolution can be easily used to identify and locate nuclear materials precisely in nuclear security and nuclear decommissioning applications. (authors)

[en] The σ-ω mixing in nuclear matter is extended to include the electromagnetic interaction between protons by mixing the photon propagator with the σ,ω propagators. The total dimesonic function, including the electromagnetic interaction is derived and its real and imaginary parts as well as the dispersion relation are calculated numerically. We find that the electromagnetic interaction has a very large effect on the real part of the dimesonic function for low momentum transfer and zero energy (or zero frequency). The imaginary part of the dimesonic function shows a resonant behavior as a function of energy for fixed momentum, where the point of the negative minimum is verified to correspond to the point in the acoustic sound spectra of the collective excitation. The dispersion relation at normal nuclear matter density indicates the existence of a completely damped acoustic sound collective excitation

[en] Wc-CO type composite powders have become satisfactory thermal spraying material necessary for wear-resistant coating badly needed in various spraying operations. China is rich in tungsten ore resources, therefore, the development of W-containing thermal spraying powders is of great importance to meet the practical needs. Two different types of WC-Co composite powders have been developed by the authors in recent years using coating and sintering methods respectively. Such powders are being used in the wear-resistant parts of aircraft engines, axles of reciprocating pumps, sealing rings, ventilators for power stations, etc. This paper is to describe the coated and sintered types of WC-Co thermal spray powders with the same chemical composition, emphasis being given to the study of characteristics of such powders before spraying and of the coatings formed under the same spraying conditions. Data and photos are obtained by means of SEM, X-ray diffraction, metallographic analysis and other methods. Physical properties including mechanical features concerning the coatings are tabulated. Theoretical explanation of the research results is also given