Performance Analysis and Modeling of Thermally Sprayed Resistive Heaters

Many processes and systems require hot surfaces. These are usually heated using electrical elements located in their vicinity. However, this solution is subject to intrinsic limitations associated with heating element geometry and physical location. Thermally spraying electrical elements directly on surfaces can overcome these limitations by tailoring the geometry of the heating element to the application. Moreover, the element heat transfer is maximized by minimizing the distance between the heater and the surface to be heated. This article is aimed at modeling and characterizing resistive heaters sprayed on metallic substrates. Heaters were fabricated by using a plasma-sprayed alumina dielectric insulator and a wire flame-sprayed iron-based alloy resistive element. Samples were energized and kept at a constant temperature of 425 °C for up to 4 months. SEM cross-sectional observations revealed the formation of cracks at very specific locations in the alumina layer after thermal use. Finite-element modeling shows that these cracks originate from high local thermal stresses and can be predicted according to the considered geometry. The simulation model was refined using experimental parameters obtained by several techniques such as emissivity and time-dependent temperature profile (infra-red camera), resistivity (four-probe technique), thermal diffusivity (laser flash method), and mechanical properties (micro and nanoindentation). The influence of the alumina thickness and the substrate material on crack formation was evaluated.     

Electrochemical recognition of chiral species using quaternary ammonium binaphthyl salts

The use of ephedrine-substituted quaternary ammonium binaphthyl salts as molecular receptors is demonstrated. The electrochemical oxidation of the receptor is affected by the binding of an analyte in solution. The binding site on the binaphthyl salt has been determined using computer modeling and confirmed using 1D and 2D NMR studies. It is shown that the sensitivity of the receptor is related to the size of the analyte. Axially chiral binaphthyl salts are shown to bind chiral analytes in a different manner and this is demonstrated using lactic and mandelic acid. The presence of a polar functional group on the analyte is also shown to have an effect on the guest-host interaction.     

Orientations of liquid crystals on mechanically rubbed films of bovine serum albumin: a possible substrate for biomolecular assays based on liquid crystals

We report the uniform planar anchoring of thermotropic liquid crystals on films of bovine serum albumin (BSA) covalently immobilized on the surface of glass microscope slides and mechanically rubbed using a cloth. The azimuthal orientations of the liquid crystals were measured to be parallel to the direction of rubbing. Following immersion and removal of these rubbed films of BSA from aqueous solutions containing either BSA, fibrinogen, lysozyme, anti-FITC immunoglobulin G (IgG), or antistreptavidin IgG, we measured liquid crystals placed onto these surfaces to largely retain their uniform alignment. In contrast, following immersion of a rubbed film of BSA into an aqueous solution of anti-BSA IgG, we observed liquid crystals on these surfaces to assume nonuniform orientations. We conclude that specific binding of anti-BSA IgG to the film of rubbed BSA erased anisotropy induced within the film of BSA by rubbing. This result suggests that the spatial scale of anisotropy within the rubbed film of BSA is comparable to or smaller than the size of the IgG molecule. Because the anisotropy within a rubbed film of a protein can be erased by specific binding of a second protein, we believe these types of substrates (rubbed films of proteins) have the potential to be useful in a variety of label-free biomolecular assays where specific binding of a target species to its ligand can be imaged through observation of the optical appearance of liquid crystal placed onto the surface.     

Do simple cells in primary visual cortex form a tight frame?

Sets of neuronal tuning curves, which describe the responses of neurons as functions of a stimulus, can serve as a basis for approximating other functions of stimulus parameters. In a function-approximating network, synaptic weights determined by a correlation-based Hebbian rule are closely related to the coefficients that result when a function is expanded in an orthogonal basis. Although neuronal tuning curves typically are not orthogonal functions, the relationship between function approximation and correlation-based synaptic weights can be retained if the tuning curves satisfy the conditions of a tight frame. We examine whether the spatial receptive fields of simple cells in cat and monkey primary visual cortex (V1) form a tight frame, allowing them to serve as a basis for constructing more complicated extrastriate receptive fields using correlation-based synaptic weights. Our calculations show that the set of V1 simple cell receptive fields is not tight enough to account for the acuity observed psychophysically.     

Divisive inhibition in recurrent networks

Models of visual cortex suggest that response selectivity can arise from recurrent networks operating at high gain. However, such networks have a number of problematic features: (i) they operate perilously close to a point of instability, (ii) small changes in synaptic strength can dramatically modify the degree of amplification, and (iii) they respond slowly to rapidly changing stimuli. Divisive inhibition, acting through interneurons that are themselves divisively inhibited, can solve these problems without degrading the selectivity of a recurrent network.     

An adaptive RAM-DFE for storage channels

A modification of the decision feedback equalizer (DFE), RAM-DFE, is presented and analyzed for use in channels with trailing nonlinear intersymbol interference, especially binary saturation-recording channels. In the RAM-DFE, a look-up table, which can be easily implemented with random access memory, (RAM), replaces the transversal filter feedback section of the DFE. The feedforward section of the equalizer remains linear. A general nonlinear Markov (or finite-state machine) model is used to model the nonlinear intersymbol interference (ISI) channel. With this Markov model, a method is introduced for computing the minimum-mean-squared-error settings of the feedforward filter coefficients and the feedback filter and look-up table contents for the linear DFE and the RAM-DFE, respectively. RAM-DFE with these settings can be significantly better than the linear DFE for channels with trailing nonlinear ISI. Globally convergent gradient-type algorithms for updating the feedforward section coefficients and the contents of the feedback table are introduced and analyzed. Results based on data taken from disk storage units are discussed     

Cleavage motifs of the yeast 20S proteasome beta subunits deduced from digests of enolase 1

The 436-amino acid protein enolase 1 from yeast was degraded in vitro by purified wild-type and mutant yeast 20S proteasome particles. Analysis of the cleavage products at different times revealed a processive degradation mechanism and a length distribution of fragments ranging from 3 to 25 amino acids with an average length of 7 to 8 amino acids. Surprisingly, the average fragment length was very similar between wild-type and mutant 20S proteasomes with reduced numbers of active sites. This implies that the fragment length is not influenced by the distance between the active sites, as previously postulated. A detailed analysis of the cleavages also allowed the identification of certain amino acid characteristics in positions flanking the cleavage site that guide the selection of the P1 residues by the three active beta subunits. Because yeast and mammalian proteasomes are highly homologous, similar cleavage motifs might be used by mammalian proteasomes. Therefore, our data provide a basis for predicting proteasomal degradation products from which peptides are sampled by major histocompatibility complex class I molecules for presentation to cytotoxic T cells.     

Tropomyosin-containing actin cables direct the Myo2p-dependent polarized delivery of secretory vesicles in budding yeast

The actin cytoskeleton in budding yeast consists of cortical patches and cables, both of which polarize toward regions of cell growth. Tropomyosin localizes specifically to actin cables and not cortical patches. Upon shifting cells with conditionally defective tropomyosin to restrictive temperatures, actin cables disappear within 1 min and both the unconventional class V myosin Myo2p and the secretory vesicle-associated Rab GTPase Sec4p depolarize rapidly. Bud growth ceases and the mother cell grows isotropically. When returned to permissive temperatures, tropomyosin-containing cables reform within 1 min in polarized arrays. Cable reassembly permits rapid enrichment of Myo2p at the focus of nascent cables as well as the Myo2p- dependent recruitment of Sec4p and the exocyst protein Sec8p, and the initiation of bud emergence. With the loss of actin cables, cortical patches slowly assume an isotropic distribution within the cell and will repolarize only after restoration of cables. Therefore, actin cables respond to polarity cues independently of the overall distribution of cortical patches and are able to directly target the Myo2p-dependent delivery of secretory vesicles and polarization of growth.