170 research outputs found
Tunneling-percolation origin of nonuniversality: theory and experiments
A vast class of disordered conducting-insulating compounds close to the
percolation threshold is characterized by nonuniversal values of transport
critical exponent t, in disagreement with the standard theory of percolation
which predicts t = 2.0 for all three dimensional systems. Various models have
been proposed in order to explain the origin of such universality breakdown.
Among them, the tunneling-percolation model calls into play tunneling processes
between conducting particles which, under some general circumstances, could
lead to transport exponents dependent of the mean tunneling distance a. The
validity of such theory could be tested by changing the parameter a by means of
an applied mechanical strain. We have applied this idea to universal and
nonuniversal RuO2-glass composites. We show that when t > 2 the measured
piezoresistive response \Gamma, i. e., the relative change of resistivity under
applied strain, diverges logarithmically at the percolation threshold, while
for t = 2, \Gamma does not show an appreciable dependence upon the RuO2 volume
fraction. These results are consistent with a mean tunneling dependence of the
nonuniversal transport exponent as predicted by the tunneling-percolation
model. The experimental results are compared with analytical and numerical
calculations on a random-resistor network model of tunneling-percolation.Comment: 13 pages, 12 figure
Understanding the basis of space closure in Orthodontics for a more efficient orthodontic treatment
Piezoresistivity and conductance anisotropy of tunneling-percolating systems
Percolating networks based on interparticle tunneling conduction are shown to
yield a logarithmic divergent piezoresistive response close to the critical
point as long as the electrical conductivity becomes nonuniversal. At the same
time, the piezoresistivity or, equivalently, the conductivity anisotropy
exponent remains universal also when the conductive exponent is not,
suggesting a purely geometric origin of . We discuss our results in
relation to the nature of transport for a variety of materials such as
carbon-black--polymer composites and RuO_2-glass systems which show
nonuniversal transport properties and coexistence between tunneling and
percolating behaviors.Comment: 6 pages, 3 figures, Added discussion on experiment
Bonding to aged surfaces: A thermally-aged epoxy
It is common experience that aged surfaces are often difficult to bond to. We report an examination of bonding to thermally-aged epoxy surfaces, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 ° C, with the ageing time varying from 2 to 8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 ° C were 0.077, 0.059, and 0.050 kJ M −2 , respectively. These compare to 0.13 kJ M −2 for a bond to an unaged surface and 0.21 kJ m −2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with ‘good’ adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was felt for some distance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44698/1/10853_2004_Article_BF00584867.pd
Evaluation of force released by deflection of orthodontic wires in conventional and self-ligating brackets
Decrease of CD68 Synovial Macrophages in Celastrol Treated Arthritic Rats
Rheumatoid arthritis (RA) is a chronic immune-mediated inflammatory disease characterized by cellular infiltration into the joints, hyperproliferation of synovial cells and bone damage. Available treatments for RA only induce remission in around 30% of the patients, have important adverse effects and its use is limited by their high cost. Therefore, compounds that can control arthritis, with an acceptable safety profile and low production costs are still an unmet need. We have shown, in vitro, that celastrol inhibits both IL-1β and TNF, which play an important role in RA, and, in vivo, that celastrol has significant anti-inflammatory properties. Our main goal in this work was to test the effect of celastrol in the number of sublining CD68 macrophages (a biomarker of therapeutic response for novel RA treatments) and on the overall synovial tissue cellularity and joint structure in the adjuvant-induced rat model of arthritis (AIA).FCT fellowship: (SFRH/BPD/92860/2013)
Semaphorin, neuropilin and VEGF expression in glial tumours: SEMA3G, a prognostic marker?
Gliomas are characterised by local infiltration, migration of tumour cells across long distances and sustained angiogenesis; therefore, proteins involved in these processes are most likely important. Such candidates are semaphorins involved in axon guidance and cell migration. In addition, semaphorins regulate tumour progression and angiogenesis. For cell signalling, class-4 semaphorins bind directly to plexins, whereas class-3 semaphorins require additional neuropilin (NRP) receptors that also bind VEGF165. The anti-angiogenic activity of class-3 semaphorins can be explained by competition with VEGF165 for NRP binding. In this study, we analysed the expressions of seven semaphorins of class-3, SEMA4D, VEGF and the NRP1 and NRP2 receptors in 38 adult glial tumours. In these tumours, SEMA3B, SEMA3G and NRP2 expressions were related to prolonged survival. In addition, SEMA3D expression was reduced in high-grade as compared with low-grade gliomas. In contrast, VEGF correlated with higher grade and poor survival. Thus, our data suggest a function for a subset of class-3 semaphorins as inhibitors of tumour progression, and the prognostic value of the VEGF/SEMA3 balance in adult gliomas. Moreover, in multivariate analysis, SEMA3G was found to be the only significant prognostic marker
The Influence of Bracket Material, Ligation Force and Wear on Frictional Resistance of Orthodontic Brackets
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