Pressed Pellet Measurements

Pressed lead azide is a two-phase dielectric system consisting of azide crystals of various dimensions and air in the pores at ambient pressure. Its dielectric strength varies with the density, crystal dimensions, thickness, etc. 

The electric fields were calculated in terms of an average field strength (k V/cm), although the electric stress could have been more highly concentrated in certain regions due to shape of the electrodes. Leopold noted that arcing sometimes occurred around the circumference of the 1.27-mm electrode, indicating a certain amount of stress concentration. 

Leopold also demonstrated that it is necessary to have contact between the explosive and the electrodes for initiation to occur. No initiations were observed with any of the test explosives when the explosive pellets were insulated from 

Pitts [31] reported a dielectric strength of 36 kV/cm for a 0.5-mm-thick lead azide pellet of 3.7 g/ml density. The explosive was confined in a block between two 4.2-mm-diam flat-bottom drill rods. A 500-pF capacitor discharge was used to determine the dielectric strength. No description was noted in the report of the type of azide investigated. 

The effect of an electric field on dextrinated lead azide pellets as a function of density and thickness was also studied [11]. The explosive was pressed in a nylon sleeve between two 0.48-mm (0.187-in) -diam, flat-surface steel rods with rounded edges. The explosive was subjected to a one-minute-on, onc-minutc-off application of voltage, increasing the voltage in 100-V increments until the sample detonated. The current was monitored continuously. 

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Figure 2 Spectrum of lichen sample. Pressed pellet, measured with a standard EDXRF spectrometer, Rh tube 30 kV, 0.2 mA, 500 s, Pd filter, Si(Li) detector concentration values of respective elements given in rg g"h...

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

Pressed pellet conductivity measurements on some of these radicals (e.g. 24 and 29) indicate room temperature conductivities of < 10-6 S cm-1.62 The low... 

Solid State Properties. Most likely as a result of the limited intermoleeular interactions of these complexes in the solid state, little investigation of their solid-state properties have been reported. However, the electrical conductivity of pressed pellets of 17 have been measured via four-point probe to give a room temperature value of 10 S cm. This relatively low value is likely a result of the lack of significantly strong t-stacking as observed in the X-ray structure. 

Infrared spectra were measured on a Perkin-Elmer model 225 spectrometer in the range 1200-200 cm-1. The calibration of the instrument was checked by measuring the frequencies of C02 vibrational bands. The infrared spectra of zeolite samples were measured as pressed pellets containing approximately 3 mg of zeolite in 300 mg of Csl. 

Pulsed-electron irradiation expts are usually conducted with accelerators charged from 0.1 to 6.0 MeV and pulse durations ranging from 3—60 nsecs. The expls are pressed pellets with thicknesses of about 0.2 of the electron range. Calorimeters are used to measure the fluences. 

Avrami et al (Ref 46) used an electrode beam app (Physics International Mod 738 Pulserad) to rapidly heat a slab of Al. The shock thus generated by rapid energy deposition was transmitted into pressed pellets of KDNBF (K dinitro-benzfuroxan). With shocks of about 0.2 microsec duration and a pellet thickness of 0.25 inch the initiation threshold measured was about 18 kbar... 

Powder samples of inorganic iron compounds can be conveniently measured with sample holders that have a sample thickness of 0.5mm and an area of 1 cm. This makes a total sample volume of 50 pL. Some Mossbauer groups prefer to measure pressed pellets, but this decision depends often on history and individual laboratory habits. If sample holders like those displayed in Figure 3 are used, one has to take special care that there are no holes in the sample. This would mean that the y-rays go through the sample holder... 

A last example to be discussed in this section is particularly instructive from a structural point of view. In the case where the two substituents in the 1,1 position are (4-methylthio-phenyl)-2- -ethenyl, the corresponding ferrocene derivative (see Fig. 8-17) still acts as a donor and forms a 1 2 CT complex with TCNQ, but the degree of electron transfer is very weak [65]. This material is non-conducting (<7rt of pressed pellets < 10" S cm" ), shows very weak signals in both ESR spectra and magnetic susceptibility measurements, and displays a CN stretching frequency typical for neutral TCNQ. As illustrated in Fig. 8-17, the solid slate structure of this CT complex is characterized by DA2 stacks, whereby the donor in the DA2 unit assumes an antiperiplanar conformation, thus being able to accomodate the... 

With a few exceptions (MacCarthy et al., 1975 MacCarthy and Mark, 1975) all infrared spctra of humic substances have been measured on dried solid samples, and the pressed-pellet method has been used almost exclusively. The mull technique has been used to a very limited extent in the study of humic substances (Ceh and Hadzi, 1956 Orlov et al., 1962 Wagner and Stevenson, 1965), and a few workers have also used the cast film method (MacCarthy and Mark, 1975 Wershaw and Pinckney, 1980). Typical infra-... 

Microelectrodes based on closed-end pH glass membranes were the first to be described in the literature (Cl). Hinke (H2) fiuther developed glass membrane microelectrodes with tip diameters of 10 im for the measurement of sodium and potassium. Various designs and approaches to the fiibri-cation of these all-glass microelectrodes have been taken over the years, and specific fabrication procedures may be found in the literature (H3, K3). Similarly, solid-state type electrodes based on pressed pellets of Ag2S with tip diameters on the order of 100 fim have been reported for the determination of Ag" ", S , I, Cl , Cu +, Br , etc. (C13). We have already discussed that such solid-state electrodes can foul when applied for direct measurements in biological systems, so the fabrication of these will not be discussed here. 

Dissolution measurements in the absence of supporting electrolyte were made on both electrochemically grown films and pressed pellets of AgCl, with similar results (7). Approach curves for tips characterized by a = 5, 12.5, and 25 /xm, constructed by plotting the normalized long-time currents, from chronoamperometric measurements, as a function of normalized tip-substrate distance are shown in Figure 34a and b. The curves cover different values of L since, in each case, measurements were made over a range of distances up to 2 /xm from the substrate surface. For comparison with the experimental data, theoretical approach curves are also shown for second-... 

What actually occurs during these final preparation procedures can be inferred from resistance measurements taken on pellets within the furnace. A DC technique with current reversal was used for these measurements. Four platinum leads were attached to the pressed pellet sample with silver epoxy. The epoxy was covered with a layer of protective ceramic paste. (8) To compensate for induced thermal emf s, an average of the forward and reversed current directions was... 

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