EFFECT OF PRESSING PRESSURE ON THE TRANSPARENCY AND MICROHARDNESS OF PRESSED SAMPLES OF ALKALI HALIDE SALTS

S. L. KASHCHUK, Yu. M. ANNENKOV

(Presented by the scientific seminar of the electrophysical laboratory of the Research Institute of Radiation Physics, TPI)

Interest in the technique of pressing alkali halide salts arose in connection with the replacement, in infrared spectral analyses of a chemical and biological nature, of immersion liquids by pressed tablets.

Studies of pressed samples have been carried out with the aim of obtaining optimal pressing regimes for producing tablets transparent in the infrared region [1–7].

At present, two directions are emerging in the study of pressed samples of alkali halide salts. The first direction is associated with attempts to obtain pressed samples whose physical properties are close to those of single crystals. The second direction is of independent scientific interest and consists in the directed alteration of the physical properties of pressed samples by applying various technologies for preparing tablets.

The purpose of the present work is to investigate the effect of pressing pressure on the transparency and microhardness of pressed tablets of NaCl, KCl, and KBr, and to select the optimal pressure at which the characteristics under study of the tablets are closest to the properties of single crystals.

On the basis of an analysis of literature data [1–15] and of our previously conducted work [16], a technology was developed for pressing tablets of alkali halide salts, and a vacuum press mold with external heating was constructed.

For pressing, a fine-grained powder was used, ground in an agate mortar and sieved through a No. 100 capron sieve. The sieved powder was dried in a vacuum furnace at a temperature of 120°C for 20 h.

The samples were pressed in a vacuum of the order of \(8 \cdot 10^{-2}\) mm Hg at a temperature of 120°C. The holding time of the samples under pressure was 10 min. The specific pressing pressure \((P_{\text{sp}})\) was varied from 3000 kg/cm² to 12000 kg/cm². The samples had the form of transparent disks 14 mm in diameter and 0.7 to 1.3 mm thick.

The transparency of the tablets was measured on an SF-4 quartz spectrophotometer in the wavelength region from 220 mµ to 1000 mµ. The transparency of the tablets was determined as a percentage of the transmission of the reference standard—air.

The transparency of the pressed samples was evaluated in accordance with the following characteristics:

1) the difference in transparency \((\Delta \delta)\) of individual specimens prepared under the same pressing conditions;

2) the limit at which maximum light transmission is established in the short-wavelength range;

3) the maximum value of light transmission.

The experimental results obtained in studying the transparency of NaCl, KCl, and KBr pellets pressed at various specific pressures are presented in Table 1. A graph of the dependence of pellet transparency on the wavelength of the radiation at various specific pressing pressures is presented in Fig. 1 for KCl pellets. Analogous results were obtained for NaCl and KBr pellets.

Table 1

Experimental transparency data for pressed NaCl, KCl, and KBr specimens

NaCl pellets prepared at \(P_{\text{sp}} = 3160\ kg/cm^2\) and \(P_{\text{sp}} = 50550\ kg/cm^2\), as well as KCl pellets prepared at \(P_{\text{sp}} = 3160\ kg/cm^2\), had a matte or spotty appearance. The transparency of such pellets was not measured.

According to the characteristics indicated above, the best properties are exhibited by NaCl pellets prepared at \(P_{\text{sp}} = 12000\ kg/cm^2\), KCl pellets at \(P_{\text{sp}} = 7580\ kg/cm^2\), and KBr pellets at \(P_{\text{sp}} = 7580\ kg/cm^2\) (Table 1).

The microhardness of the pressed specimens was measured on a PMT-3 instrument. Figure 2 presents the dependences of microhardness on specific pressing pressure for pressed specimens of NaCl, KCl, and KBr.

The microhardness of NaCl and KCl pellets changes with increasing specific pressure according to a curvilinear dependence, with a minimum at \(P_{\text{sp}} = 10100\ kg/cm^2\).

The microhardness of NaCl and KCl pellets prepared at \(P_{\text{sp}} = 10100\ kg/cm^2\) is close to the microhardness of NaCl and KCl single crystals.

The microhardness of KBr tablets changes hardly at all with increasing specific pressing pressure and, for all samples, is close to the microhardness of a KBr single crystal.

The decrease in the microhardness of NaCl and KCl tablets with an increase in specific pressure up to a certain value is associated with an improvement in the packing structure of the pressed grains. A further increase in microhardness with a subsequent increase in pressure is associated with the onset of recrystallization of the structure and with an increase in stresses in the pressed compact.

Fig. 1. Dependence of transparency (in percent transmittance relative to air, %) on the radiation wavelength—\(\lambda[\mu]\)—for KCl tablets pressed at:

\[ 1 — P_{\mathrm{sp}} = 5050 \frac{\mathrm{kg}}{\mathrm{cm}^{2}};\quad 2 — P_{\mathrm{sp}} = 7580 \frac{\mathrm{kg}}{\mathrm{cm}^{2}}; \]

\[ 3 — P_{\mathrm{sp}} = 10100 \frac{\mathrm{kg}}{\mathrm{cm}^{2}};\quad 4 — P_{\mathrm{sp}} = 12000 \frac{\mathrm{kg}}{\mathrm{cm}^{2}} \]

Fig. 2. Dependence of microhardness \(H_\mu[\mathrm{kg}/\mathrm{mm}^{2}]\) on the specific pressing pressure—\(P_{\mathrm{sp}}\) \([\mathrm{kg}/\mathrm{cm}^{2}]\)—for NaCl, KCl, and KBr tablets.

Conclusions

1. The highest-quality pressed samples are obtained as a result of vacuum pressing at a temperature of \(120^\circ\mathrm{C}\).

2. The properties of pressed samples of alkali-halide salts depend on the specific pressing pressure.

3. For salts of different chemical composition, optimal pressing regimes have been selected under which the properties of the tablets are closest to those of single crystals.

NaCl tablets should be pressed at \(P_{\mathrm{sp}} = 11000\ \mathrm{kg}/\mathrm{cm}^{2}\), and KCl and KBr tablets at \(P_{\mathrm{sp}} = 7580\ \mathrm{kg}/\mathrm{cm}^{2}\).

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