Transient measurements, insulating

The upper and lower dashed curves correspond respectively to the limits of a conductor (Kl s —> °°) and of an insulator (Kb s = 0) Equation (6) gives a quantitative estimate of the effect of tip size and rate constant on the transient. The faster the electron transfer, the smaller the electrode has to be to perform a transient measurement of the rate constant. 

A newer method is the transient hot-wire method, where an electric current is passed through a metal wire immersed in the fluid. The resistance of the wire is affected by its temperature, which in turn is affected by the dissipation of heat from the wire s surface, which depends on the thermal conductivity of the fluid. These instruments require sophisticated data analysis, but that is no longer an obstacle with the ready availability of personal computers. The absence of convection is relatively easy to verify. The best research instruments can achieve an accuracy of better than 1%. Measurements on conducting fluids (such as polar liquids) are more difficult because of the need to electrically insulate the wire. Other geometries, such as needle-shaped cylinders and thin strips, are also sometimes used for transient measurements. 

Heatshield thickness and weight requirements are determined using a thermal prediction model based on measured thermophysical properties. The models typically include transient heat conduction, surface ablation, and charring in a heatshield having multiple sublayers such as bond, insulation, and substmcture. These models can then be employed for any specific heating environment to determine material thickness requirements and to identify the lightest heatshield materials. 

A number of variations of the transient hot-wire method have been devised, and an optical method to detect the temperature rise has been used. A modified transient hot-wire technique using a mercury-incapillary probe was introduced by Nagashima et al., in which a thin mercury thread was used as a heater-thermometer and the capillary wall as an insulator. Using this method, they measured the thermal conductivity in mixture systems such as (Na, K)N03, (Li, Na)N03, and HTS(KN03-NaN03-NaN02, 44-7-49 mol.%). ... 

Detonation Velocity by Metallic Transition of Sulfur. Joigneau Thouvenin (Ref 1) reported a large increase in the elec conductivity of cryst sulfur when it was subjected to high transient pressure, but detected no sudden or discontinous transition to metallic condition. It was inferred from their results that a modified system should permit the use of sulfur as an active element of a pressure transducer for measurements in the kilobar range Hauver (Ref 2) prepd a modified system using a thin disc of sulfur, 0.00 50 inch thick and 9/32 inch in diam, insulated in Teflon. A number of tests were performed... 

In Table IV we present Eai and Ei0 data on two important deep centers in GaAs, Cr, and O (EL2). The results from three different laboratories are included, but no attempt was made to show everything available in the literature. It is clear that neither the Eai results nor the Ei0 results agree well for Cr, but are not too bad for O. In contrast, the TDH measurements of El0, shown in Table II, are much more consistent. It should be noted that the TDH samples (Table II) were semi-insulating, whereas the emission-spectroscopy samples (Table IV) were conducting in order that capacitance transient (DLTS) experiments could be performed. The PITS and OTCS techniques applied to these samples would have been unable to clearly distinguish between hole and electron traps. 

In the absence of space charges, but where an electric field is applied across the insulating sample, the current transient reflects the induced charges at the electrode/insulator interface. Since these charges occupy a negligible depth, the profile of the current peaks reflects the profile of the pressure pulse itself. This enables a calibration of the measurement system to be made. 

Fig. 12.7 The transient current, i.e. the current as a function of the time following a light pulse in crystals of (Me-Me-DCNQI)2Cu as a mixture of/ig/cfe (see Fig. 12.5), following optical excitation with a ps light pulse. The measurement was carried out at three different temperatures in the insulating region OPl and OPS near the phase transition at higher and lower temperatures (reentry, compare...

This work describes a new probe of dynamic processes accompanying the tribological loading of metal/polymer interfaces. We instrument the polymer substrate and a conducting stylus to measure the transient electrical currents generated as the stylus is moved across the substrate under normal load. Simultaneous measurements of the lateral and normal forces on the stylus are also performed. Both sets of measurements are readily made on ms to is time scales. To date, we have measured currents accompanying the abrasion of several insulators. 

Measurement of Membrane Resistance. For a given value of Cm, HI is a function of the potential across the semiconductor and insulator (Vt) The equivalent circuit can therefore be simplified as shown in Figure 4. For bias potentials near I pip, HI follows 4 approximately linearly. Because no significant direct current can flow through the insulator, Rm doesn t enter into the DC characteristics of this system, but only affects transient responses to changes in bias potential. Let us consider the case where the bias potential is stepped with time. Immediately after the potential step, the new bias potential will be distributed across the capacitances with the time constant Ti (Equation 5). Ci is the total capcitance of the insulator in contact with electrolyte and Co is the total capacitance of the depletion region under the electrolyte (therefore Ct is calculated for the area of the semiconductor in contact with solution and not just the illuminated area as in Equation 4). Re is due to the resistance of the electrolyte. 

Boron Thermopiles. The boron thermopiles are used for an additional check on neutron flux level. They are located, in the graphite. (VG-27, 28, 42, 44, 56 see Fig.- 3.F) all at the same horizontal level. These instruments consist of a large number of thermocouple junctions in series, with alternate junctions coated with boron. The coated junctions become warm owing to absorption of neutrons, thus yielding a net voltage which is a measure of neutron flux. The instrument case is about H in. in diameter by 6 in. long. The approximate sensitivity is 6 mv in a flux of 10. Internal resistance is of the order of 3 to 4 ohms. By proper mounting and thermal insulation of the thermocouples the instruments may be made quite insensitive to ambient temperature (say, 40 /xv/°C). For transient conditions the time constant is about 1 sec. 

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