137 research outputs found
Spectrum and polarization of laser light scattered by solids
Laser light scattering from yttrium-iron garne
Growth kinetics of nuclei formed from different binders and powders in vertical cylindrical mixing devices
peer-reviewedGranulation is the process of forming large aggregates from fine particles using a high shear mixer. This method is used in several industries from pharmaceuticals to chemical and fertilizer production. This research will study the effect of four process variables: speed of mixer rotation in the range 100–200 rpm, powder bed mass (25–40 g), mass of the initial nucleus (0.6–2 g), and binder viscosity (water, carboxymethyl cellulose (CMC) solutions with concentrations in the range 0.5–20 g/L) on single nuclei growth kinetics in low mixing devices. The powders under study were: lactose, tea, sugar, starch, and limestone. The results show the initial size of nuclei, the initial mass of the powder bed and binder viscosity and speed of rotation all influence the rate of nuclei growth. Analysis of the stokes deformation number of the nuclei show that growth rate of the nuclei decreases as the deformation number increases whilst the percentage gain in mass of the nuclei increases with increasing deformation number. The binder viscosity was shown to have the biggest influence of the growth rate of the nuclei. Results show that difference in powder density also has an effect on the growth kinetics of nuclei. The initial position of nuclei was also shown to influence the nuclei growth rate; the closer the starting position of the nuclei to the wall of the vessel the slower the growth rate
Assessment of intragranular and extragranular fracture in the development of tablet tensile strength
When a tablet is compacted from deformable granules and then broken, the fracture plane may cleave granules in 2 (intragranular fracture) or separate neighboring granules (extragranular fracture). In this study, a novel method was developed to quantify the extent of intragranular versus extragranular fracture by compacting tablets from multicolored ideal granules and evaluating fracture surfaces. The proportions of intragranular and extragranular fracture were quantified and modeled in light of a new metric; the deformation potential, Δ, reflecting the solid fraction increase as an initial granule bed is compressed into a final tablet. Results show that a measurable tablet strength is achieved at Δ > 0.18, but intragranular fracture is not observed until Δ > 0.21. At very large Δ, tablets experience almost exclusively intragranular fracture, yet the tablet tensile strength is considerably lower than that of a tablet compacted from raw powders versus precompacted granules. Thus, secondary compaction of granules appears to weaken the granule matrix, leading to reduced tablet tensile strength even in the presence of strong extragranular bonding
Binder-treated segregation-free aluminum alloy powders
Two paraffin waxes with different melting points and mixed at a specific mass ratio were used as a binder for premixed aluminum alloy powders. Dusting, segregation, and part-to-part variability were significantly reduced and powder flow improved compared to untreated powder In addition, the green density of compacts fabricated from the binder-treated powders was higher than that of the untreated powders. Sintered mechanical properties were not significantly affected by the addition of the binder Air-drying alkyd binders and water-based wax emulsions were also tested, but the results were less satisfactory
Dynamics of Weak First Order Phase Transitions
The dynamics of weak vs. strong first order phase transitions is investigated
numerically for 2+1 dimensional scalar field models. It is argued that the
change from a weak to a strong transition is itself a (second order) phase
transition, with the order parameter being the equilibrium fractional
population difference between the two phases at the critical temperature, and
the control parameter being the coefficient of the cubic coupling in the
free-energy density. The critical point is identified, and a power law
controlling the relaxation dynamics at this point is obtained. Possible
applications are briefly discussed.Comment: 11 pages, 4 figures in uuencoded compressed file (see instructions in
main text), RevTeX, DART-HEP-94/0
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Synchrotron X-Ray Study of Novel Crystalline-B Phases in Heptyloxybenzylidene-Heptylaniline (7O.7)
This paper reports an x-ray diffraction study of structures and restacking transitions within the B phases of heptyloxybenzylidene-heptylaniline. The system evolves from a hexagonal close-packed structure, through intermediate orthorhombic and monoclinic phases, to a simple hexagonal structure. The monoclinic phase has a temperature-dependent shear which transforms the system from orthorhombic to hexagonal. The latter three phases exhibit a single-q⃗ sinusoidal modulation of the molecular layers.Engineering and Applied Science
Microstructure of single-droplet granules formed from ultra-fine powders
A quantitative analysis of variations in granule microstructure based upon changes in primary particle size and bed preparation is presented. The granule microstructures are obtained using X-Ray Computed Tomography (XRCT). An algorithm is developed to measure the number and size of macro-voids (pore space with volume equivalent size greater than or equal to 30 μm or 3 times the primary particle size). Four size fractions of alumina, ranging in primary particle size from 0.5 μm to 108 μm, are sieved using three different sieve sizes to create static powder beds from which single-droplet granules are produced. The analysis shows that large macro-voids exist in ultra-fine powders (0.1–10 μm). The macro-voids take up to 7% of the granule volume and the largest macro-voids are 200–700 μm in volume equivalent size. Changing the sieve preparation changes the size and total volume of macro-voids. In contrast, there are very few macro-voids in granules formed from coarser powders. This study shows that micron sized powders have the opportunity to form complex structures during granulation and that the handling history of the materials should receive greater scrutiny than it currently gets
Critical structure factors of bilinear fields in O(N)-vector models
We compute the two-point correlation functions of general quadratic operators
in the high-temperature phase of the three-dimensional O(N) vector model by
using field-theoretical methods. In particular, we study the small- and
large-momentum behavior of the corresponding scaling functions, and give
general interpolation formulae based on a dispersive approach. Moreover, we
determine the crossover exponent associated with the traceless
tensorial quadratic field, by computing and analyzing its six-loop perturbative
expansion in fixed dimension. We find: ,
, and for respectively.Comment: 27 page
Morphology of supported polymer electrolyte ultra-thin films: a numerical study
Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM
fuel cell catalyst layers has significant impact on the electrochemical
activity and transport phenomena that determine cell performance. In those
regions, Nafion can be found as an ultra-thin film, coating the catalyst and
the catalyst support surfaces. The impact of the hydrophilic/hydrophobic
character of these surfaces on the structural formation of the films has not
been sufficiently explored yet. Here, we report about Molecular Dynamics
simulation investigation of the substrate effects on the ionomer ultra-thin
film morphology at different hydration levels. We use a mean-field-like model
we introduced in previous publications for the interaction of the hydrated
Nafion ionomer with a substrate, characterized by a tunable degree of
hydrophilicity. We show that the affinity of the substrate with water plays a
crucial role in the molecular rearrangement of the ionomer film, resulting in
completely different morphologies. Detailed structural description in different
regions of the film shows evidences of strongly heterogeneous behavior. A
qualitative discussion of the implications of our observations on the PEMFC
catalyst layer performance is finally proposed
On the Strength of First Order Phase Transitions
Electroweak baryogenesis may solve one of the most fundamental questions we
can ask about the universe, that of the origin of matter. It has become clear
in the past few years that it also poses a multi-faceted challenge. In order to
compute the tiny primordial baryonic excess, we probably must invoke physics
beyond the standard model (an exciting prospect for most people), we must push
perturbation theory to its ``limits'' (or beyond), and we must deal with
nonequilibrium aspects of the phase transition. In this talk, I focus mainly on
the latter issue, that of nonequilibrium aspects of first order transitions. In
particular, I discuss the elusive question of ``weakness''. What does it mean
to have a weak first order transition, and how can we distinguish between weak
and strong? I argue that weak and strong transitions have very different
dynamics; while strong transitions proceed by the usual bubble nucleation
mechanism, weak transitions are characterized by a mixing of phases as the
system reaches the critical temperature from above. I show that it is possible
to clearly distinguish between the two, and discuss consequences for studies of
first order transitions in general. (Invited talk given at the ``Electroweak
Physics and the Early Universe'' workshop, Sintra, March 23-25, 1994.)Comment: 16 pages, 4 figures not included (can be obtained from
hep-ph/9403310, or by request) RevTeX, DART-HEP-94/0
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