Graduate Thesis Or Dissertation
 

Electrical conductivity of additively colored potassium chloride crystals

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  • When the dc conductivities of various pure KC1 crystals were compared it was found that crystals grown under an HC1 atmosphere, from salt purified by ion exchange, exhibited lower knee temperatures than any of the crystals studied. These knee temperatures were found to be approximately 350°C. From this the total divalent cation concentration was estimated to be 1.8 x 10¹⁴cm⁻³. It was also found that no absorption bands were present, in ion exchange purified crystals, in the region between 185 mμ and 300 mμ. The conductivities of colored KC1 crystals were found to be much greater than for uncolored crystals. Above approximately 400°C the current decreased with time according to the expression i/i₀ = 1 - c(t-t₀), where t₀ was the time when the current was i₀. c was found to depend on temperature according to the expression c = 17,000 exp (-1.12 ev /kT) sec⁻¹. The decrease in current was associated with a loss of F-centers from the crystals. Optical scanning of partially bleached crystals showed that, during bleaching, a definite boundary was formed between a colorless region at the surfaces and a colored region in the interior. The electronic transport number was determined to be 0.96 at 400°C. Thus the increased conductivity was attributed to ionized F-centers. Plots of log₁₀σ vs. 1000/T were linear with two different slopes, depending on the history of the sample. If gold contacts were evaporated onto freshly cleaved crystals rectification was observed. The rectification could be eliminated by either heating the colored crystals for five minutes, at temperatures greater than approximately 500°C, after applying the contacts, or by sanding the crystals prior to applying the contacts. Accompanying the loss of rectification was a decrease in the conductivity and an increase in the activation energy for conduction, from 1.13 ev to 1.22 ev. Plots of σ, at a given temperature, vs. n[subscript F]¹[superscript /]² were found linear. From this, and the theory of rectifying contacts, a calculation of the thermal ionization energy for F-centers was made. The value obtained was 2.04 ev. Conductivity and electronic transport number measurements were made on CaC1₂-doped KC1 crystals. From observations that the conductivity was not changed significantly after coloring and that the electronic transport number was less than 0.15, it was determined that the calcium remained divalent after coloring.
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