High impedance materials

These guns were developed to attain much higher impact velocities than those available with powder guns (Crozier and Hume, 1957 Curtis, 1962). Peak velocities in the 7-8 km/s range can be routinely attained, with peak shock pressures approaching 1 TPa in high impedance materials. 

Of course we know that po is constant U is not. The shock velocity increases with pressure. However, the product poUor Z, although increasing with pressure, increases rather slowly, and we can consider it to be somewhat constant within reasonable ranges of interest. It is constant enough to let us differentiate between a low-impedance material and a high-impedance material. When a shock passes from a low to high impedance across a material interface, the shock pressure will be increased the converse also holds. 

Ultrasound propagating from a material of low acoustic impedance on top of a high impedance material, thus generating a positive reflection. 

Most modem potentiostats provide at least four connections to the cell. These connections typically consist of a counter electrode (CE) that provides current to the ceU, a working electrode (WE) that provides measurement of the current through the cell, and at least two reference electrode inputs (RE) for voltage measurement. Potentiostats that have 4-terminal connections are capable of 2, 3 or 4-terminal testing of electrochemical cells (Figure 3.2.2). The 2-terminal test technique is used mainly for the measurement of high impedance materials where the impedance of cables is not significant (see Section 3.2.2.3). 

For measurement of high impedance materials, 2-terminal test connections to the sample are usually used. This is because the impedance of the sample is generally much greater than the impedance of the test cables therefore, any errors introduced by the cables are unlikely to significantly affect the sample measurement results. This is true at low frequency in particular but at high frequency, specialist techniques are often needed to minimize errors due to cables and these will be discussed later in this section. 

The shock pressures attainable with direct explosive contact depend on the shock impedance (shock velocity times material density) of the specimen material, and on the explosive energy of the contacting explosive. High-energy explosives placed directly on high-shock impedance materials can produce shock pressures of several tens of GPa. 

Noble metal connections can reduce the corrosion to an "acceptable" level. This assumption is not true for leads which enter the package from sensors such as micro-electrodes which are characterized by relatively high impedances. The trend for neuroprosthe-tio devices is towards closed-loop control in which the use of high impedance bioelectric sensors will be common. In addition, differing potentials within multi-circuit cables can result in corrosion even when the conductors are fabricated from highly corrosion resistant materials such as MP35N. 

In addition, it has been observed in all of the materials Investigated that once the film is in the "ON" state it will remain in that state as long as an external field is applied. In every case studied, the film eventually returned to its initial high-impedance state after the applied field was removed. It was also found that the time required to switch back to the initial state appeared to be directly proportional to the film thickness, duration of the applied field, and the amount of power dissipated in the sample while in this state. 

Many materials have the properties of low conductance (high impedance) and low loss. These materials are often referred to as dielectrics. In addition, many materials not normally considered as dielectrics exhibit these properties. It is well known that dielectric methodologies are a good test for the study of molecular and cationic mobilities in materials [88,101-134],... 

Polarisation effects at electrodes become most prominent when the material of a specimen shows some appreciable bulk conductivity. Characteristically, there is an apparent increase in the relative permittivity at low frequencies. The anomaly originates in a high-impedance layer on the electrode surface. This may be caused by imperfect contact between the metal electrode and the specimen, aggravated by the accumulation of the products of electrolysis, etc. At low frequencies there is sufficient time for any slight conduction through the specimen to transfer the entire applied field across the very thin electrode layers, and the result is an enormous increase in the measured capacitance. For a purely capacitive impedance Ce at the electrodes, in.series with the specimen proper (geometrical capacitance C0), Johnson and Cole (1951) showed that the apparent relative permittivity takes the approximate form ... 

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