Vacuum Annealed Samples Research Articles

Heat Capacity in the Normal and Superconducting States and Critical Field of Tantalum

The heat capacity of a vacuum-annealed sample of tantalum has been measured in the normal and superconducting state from 1.3 to 25\ifmmode^\circ\else\textdegree\fi{}K. In the normal state the heat capacity ${C}_{N}$ can be represented by the expression ${C}_{N}=0.00136T+464.4{(\frac{T}{\ensuremath{\theta}})}^{3}$, where $\ensuremath{\theta}$ varies from approximately 255 degrees at the lowest temperature to 220 degrees at the highest. The zero-field transition temperature, ${T}_{c}$, and the critical field ${H}_{c}$ were determined using a calorimetric method. ${T}_{c}$ was found to be 4.39\ifmmode^\circ\else\textdegree\fi{}K. The width of the transition was approximately 0.03\ifmmode^\circ\else\textdegree\fi{}C. The measured critical field was within experimental error comparable to that calculated from the heat-capacity data. The tantalum sample therefore exhibited the properties of an ideal superconductor characteristic of the so called soft superconductors. The critical field at the absolute zero, from an extrapolation of the critical field data, was found to be 780 gauss.

Photolytic Darkening and Electronic Range in AgCl

Photolytic darkening and electron range were investigated for a series of silver chloride crystals cut from a single boule which was grown in vacuum by the Bridgman technique. The effect of annealing atmosphere on both the initial darkening rate and electron range is discussed. Darkening produced by absorption of light in the tail of the fundamental band is shown to have a volume and a surface component. The volume darkening produced in a vacuum-annealed crystal is capable of absorbing but not scattering light. The darkened crystal can be bleached selectively with high efficiency by exposure to light near the peak of the induced absorption band. Difficulty is found with interpreting the results entirely in terms of a classical theory for light absorption by colloidal particles suspended in a dielectric medium. Crystals annealed in air darken quite differently then vacuum-annealed samples and crystals heated in chlorine show little sensitivity to light. Somewhat smaller but regular differences in low-temperature electron range were found for the various crystals heat treated in different annealing atmospheres.