Super-threshold

Comparing the type of information obtained on suspensions with that obtained on composites gives useful insight into the types of mechanisms that control creep of ceramic matrix composites. The very large increase in creep resistance of dense particulate composites, i.e., more than 65vol.% particles, suggests that the particle packing density is above the percolation threshold. Creep of particulate composites is, therefore, controlled by direct interparticle contract, as modified by the presence of relatively inviscid matrices. Mechanisms that control such super-threshold creep are discussed in Section 4.5. 

Figure 2 shows I-V curves for clean and contaminated samples at 100% RH. Within a range of 1.25 volts, the resistances of both samples were 10 ohms. Above a threshold of 1.25-1.5 V, the contaminated sample exhibited a sharp leakage current increase, and the resistance dropped to 10 ohms. The clean specimen exhibits a similar, but much smaller leakage current increase, corresponding to a super-threshold resistance of 10 ohms. At positive bias levels from 8 to 10 V, the current reaches a plateau of 600 nA. 

The effect of DC bias on a contaminated sample at 100% RH is shown in Figure 5. At bias levels corresponding to threshold and super-threshold levels for electrochemical reactions, the impedance spectrum shows the capacitive loop that intersects the real axis at low frequency (.1 Hz). Zero-DC-bias data, which are not shown, form a similar arc that is large compared to the scale of this plot. This behavior is modelled by a parallel RC circuit, whose resistance decreases from 1 x 10 to 1.6 x 10 and whose capacitance remains constant at approximately 30000 pF, as DC bias is raised from 0 to 3.0 V. The resistances agree with those measured in DC leakage current experiments. The capacitances are 100 times larger than those measured on the clean sample at 100 % RH. 

Fig. 1.1. Response (right) of an excitable system to different stimuli (left). The system has a stable fixed point. Panel a) A sub-threshold perturbation generates a small system response. Panel b) A super-threshold perturbation initiates an excitation loop. Panel c) The system can not be excited by a super-threshold perturbation applied during the refractory state. Panel d) Two successive super-threshold perturbations generate excitations only if both are applied to the system in the rest state.

In the upper row we see the excitable regime. The solid lines represent the nullclines of the system, the dashed line a typical trajectory. Each dash represents a fixed time interval, i.e. where the system moves faster through phase space the dashes become longer. The system possesses one fixed point (intersection of the nullclines) which is stable. Small perturbations decay. A super-threshold perturbation leads to a large response (spike) after which the system returns to the fixed point. After that a new perturbation is possible if the system from outside is brought again over the threshold. 

Fig. 1.3. Trajectories of the FitzHugh-Nagumo model for sub- and super-threshold perturbations (red and blue curves respectively). Bach point plotted at constant time intervals ti = ij-i At witli At = 0.005. Kinetic parameters 6 = 1.4, a = 1/3 and 7 = 2. Left high time scale separation for = O.Ol. Right low time scale separation for = 0.1.

Figure C3.6.7 Cubie (li = 0) and linear (( = 0) nullelines for the FitzHugh-Nagumo equation, (a) The excitable domain showing trajectories resulting from sub- and super-threshold excitations, (b) The oscillatory domain showing limit cycle orbits small inner limit cycle close to Hopf point large outer limit cycle far from Hopf point.

Some pheromones of social insects which function as excitants and attractants at low (=physiological) concentrations are powerful repellents when present at super threshold concentrations (27), The ability of these insects to be deterred by high levels of their own pheromones can provide a means of detecting new repellents for eusocial arthropods. Beyond this consideration, it is possible that these "pheromonal repellents might serve as repellents for a wide range of insects. 

As discussed in Section 1.2.1, for such systems near the percolation threshold Pc the nearest-neighbour occupied bonds (or sites) form a statistically defined super-lattice , made of tortuous link-bonds (of chemical length Lc) crossing at nodes separated by an average distance the percolation correlation length (see Fig. 1.3 of Chapter 1). The external stress... 

Jensen One thing we must realize abouty is that it acts as a threshold variable for creativity or super-performance in many other fields. You don t find any outstanding musicians with low g. You may find idiot savants who can play the... 

The threshold of the pulse energy to induce the laser tsunami is relatively low for a femtosecond laser compared with nanosecond and picosecond lasers. The laser tsunami expands to a volume of (sub pm)3 around the focal point, when an intense laser pulse is focused into an aqueous solution by a high numerical aperture objective lens. When a culture medium containing living animal cells is irradiated, they could be manipulated by laser tsunami. Mouse NIH 3T3 cells cultured on a substrate can be detached and patterned arbitrarily on substrates [36]. We have also demonstrated that the laser tsunami is strong enough to transfer objects with size of a few 100 pm [37], which is impossible by conventional optical tweezers because the force due to the optical pressure is too weak. In addition we demonstrated for the first time the crystallization of organic molecules and proteins in their super-saturated solution by laser tsunami [38-40]. 

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