Pressed disc

Laser vaporization of a composite pressed disc of graphite and BN using He as carrier gas, followed by mass spectrometric analysis, gave a range of clusters with even numbers of atoms from less than 50 to well above the peak... 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

Porous materials, such as silica and alumina, have thermal diffusion lengths of approximately 10 m, which is much less than the typical thickness of pressed discs. The small thermal diffusion length gives photoacoustic spectroscopy a larger dynamic range than transmission methods when applied to powdered samples. An additional advantage is the ease of sample preparation, since photoacoustic spectroscopy uses powdered samples with no special preparation required. 

Sample Preparation Pressed Disc of Mull Loose Powder... 

The densities of polymers can be determined by the pyknometer technique or by the flotation method. In the pyknometer technique the liquid volume displaced by the polymer sample is determined by weighing. Most polymers have a density larger than that of water, which can, therefore, be used as the liquid. Polymers in the form of powders or pressed discs tend to adsorb or occlude air bubbles, which can lead to serious errors. This can be largely prevented by careful degassing of the pyknometer and polymer sample under vacuum before filling with liquid, and/or by addition of a small amount (0.1%) of commercial detergent to lower the surface tension of the water. 

Figure 7. Comparison between responsivities of single-crystal n-Ti02, pressed-disc n-Nb02, and pressed-disc n-Ti0. 95Nb0. os02. Electrodes are biased at - -1.0 V relative to SCE (pH = 10).

In the pressed disc technique a known weight of sample is intimately ground with pure, dry potassium bromide and the mixture inserted into a special die and subjected to pressure under vacuum. The concentration of sample in the disc is usually in the region of 1.0 per cent. The disc so produced may be mounted directly in the sample beam path of the spectrophotometer and the spectrum recorded. This method has the advantage that the spectrum so produced is entirely due to the sample since pure dry potassium bromide is infrared transparent in the 2-25 /xm region. To eliminate the possibility of impurities in the potassium bromide, however, a blank disc (no sample) can be made and mounted in the reference beam path of the spectrophotometer. Care should be taken to ensure that both discs are of equal thickness otherwise inverse peaks may occur if the potassium bromide is damp or impure, and this will be particularly noticeable if the reference disc is thicker than the sample disc. 

Fig. 8 Comparion of geometric structure of an ideal auxiliary phase (a) with those prepared by current techniques, such as in-situ formation (b), mechanically pressed discs (c), physical vapor deposition (d), and melting and quenching (e).

For IR pressed disc spectra, check against mull spectra vary grinding procedure. 

V. G. Farmer. Spectrochim, Acta 8, 374-89 (1957). IR pressed discs and the effect of grinding, phenols, acids, glucose. 

Since there is no suitable solvent for film casting of PP at ambient temperature, discs, 1 cm In diameter and approximately 230 mp In thickness, were prepared by placing 80 mg of the polymer In a continuously evacuated die and compressing to 20 tons/sq cm for 4 minutes In a Perkln-Elmer hydraulic press. Discs prepared in this way are opaque. 

The infrared spectra were obtained using a commercial FTIR spectrometer (Mattson Centauri). The studies were performed in transmission mode, with the catalyst in the form of a pressed disc, using an environmental cell where the temperature, pressure, and gas composition could all be controlled [7]. All spectra were recorded at 4 cm resolution with the co-addition of 300 scans using an MCT detector. 

Zeolite films were also explored for their use in polenliomctric reactions.[93] The authors determined the potentiometric response of zeolite-modified electrodes towards cations in aqueous phase. Three different preparation routes were used for the formation of the zeolite films zeolites Y (FAU), A (LTA) and mordenite (MOR) were pressed into discs, sodalite (SOD) was grown in a free-standing membrane, and for the pressed discs of LTA a secondary growth phase was added in order to heal defects in the pressed discs. The authors could demonstrate size-selective behaviour in these systems, i.e., cations with diameters exceeding those of the zeolite window openings resulted in no detectable potential response. 

Infrared spectra have been recorded on a NICOLET MX-1 Instrument (Caen) or on a DIGILAB FTS-15E one (IFP). Self supporting pressed discs (ca. 5 mg cm 2) have been activated by heating under vacuum at 723 K for 12 hours. Pyridine (102 Pa at equilibrium) was introduced at room temperature, then immediately evacuated at the same temperature and at 423 K to eliminate physisorbed species. Analysis of the spectra then obtained allows for the determination of OH bands insensitive to pyridine. Subtraction of spectra after pyridine desorption from those obtained before pyridine adsorption evidences well bands due to acidic OH groups. 

I.r. studies have been made at 10 and 310 K of caesium cyanate and of NCO isolated in caesium iodide by employing the alkali-metal halide pressed disc technique. On replacing CsCNO by KCNO, heating for several hours at 673 K was necessary to encourage diffusion away of ions and prevent absorptions due to KCNO appearing with aging. ... 

Cody, R. D. G. L. Thompson, 1976. Quantitative X-ray diffraction analyses of clays using an orienting internal standard and pressed disc of bulk shale samples. Clays Clay Mineral. 24 224-231. 

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