Diffusion Of Barium Research Articles

Study of Diffusion of Barium in Various Carbons by Electron Microprobe

Study of Diffusion of Barium in Various Carbons by Electron Microprobe

The Diffusion of Barium in a Rutile Single Crystal

Abstract 1) Microscopic and radiotracer studies have been made upon the diffusion of barium into a rutile single crystal. 2) In the microscopic observation, the diffusion layer of barium oxide in the rutile single crystal was confirmed to liquefy above 1300°C due to the formation of a eutectic mixture between both constituents. Below 1300°C solid phase diffusion prevails, and the diffusion layer consists of polycrystalline of a random orientation. 3) The diffusion coefficients of barium oxide in a rutile single crystal were measured by means of the radiotracer method using 133Ba. The diffusion coefficients obtained were 3.5×10−14∼2.4×10−13cm2/sec. in the range of 1100∼1200°C and 1.1×10−12 cm2/sec. at 1300°C. Between 1100°C and 1200°C, small differences seemed to be observed for the values obtained according to the direction of the diffusion in the crystal, but this is not conclusive. The large value of the diffusion coefficient at 1300°C was considered to be due to the liquefaction of the diffusion layer. The energy of activation was calculated to be 43 kcal./mol. (parallel to the c-axis) and 59 kcal./mol. (perpendicular to the c-axis).

Optical Absorption and Photoconduction in the Visible and Near Infrared in Single Crystals of BaO

Optical absorption studies of single crystals of BaO indicate the presence of absorption bands at 0.8 and 1.4 ev, induced by ultraviolet or x-ray irradiation at about - 160\ifmmode^\circ\else\textdegree\fi{}C. These bands are enhanced about five-fold upon heating a crystal in air to about 1600\ifmmode^\circ\else\textdegree\fi{}C and quenching.Crystals heated in barium vapor have a blue color and have an absorption maximum at 2.0 ev. Crystals heated in aluminum, magnesium, and calcium vapors have similar absorption curves, suggesting that either interstitial barium or oxygen vacancies are responsible for the 2.0-ev absorption. These results, taken together with the results of Redington on diffusion of radioactive barium and of Sproull, Bever, and Libowitz on the diffusion of color centers in BaO crystals, favor the latter imperfections as the cause of the coloration.Photoconduction studies, used mainly as a sensitive measurement of optical absorption, show the presence of energy levels at 2.0 and 2.6 ev in addition to those at 0.8 and 1.4 ev. Extensive x-ray irradiation greatly increases the 2.0-ev level photoconduction relative to that of the other bands.

Diffusion of Barium in an Oxide-Coated Cathode

The diffusion of Ba through activated BaO–SrO cathode coatings was studied as a function of temperature. Most of the radioactive tracer material diffused according to a dependence of log10D vs 1/T similar to that obtained by Redington for diffusion of Ba in single crystals of BaO. The activation energy was 4.1±0.6 ev above 1280°K and 0.40±0.07 ev below that temperature.

Physical Processes in the L Cathode

The activity of this barium-on-tungsten cathode is maintained by dispensing barium from a pellet of barium compound. The dispensing mechanism is shown to be controlled by surface diffusion of barium over tungsten. Despite incomplete coverage of the emitting surface with barium, and almost complete absence of streaming of active material out through pores in the surface material, evaporation of barium is sufficient to be of concern in certain applications.

Photoelectric Properties of Composite Surfaces at Various Temperatures and Potentials

Experiments designed to determine the variation of photoelectric current with temperature and plate potential for barium photoelectric cells are described. The technique used in making the cells is also given. A new method, employing interrupted illumination, a transformer coupled amplifier and a cathode ray oscillography, is developed for measuring the photoelectric current. In this way the thermionic current and currents of slow responses are completely eliminated and only the a.c. component of the photo-current is amplified.Experimental results. With constant plate voltage the photoelectric current increases gradually with temperature and at about 600\ifmmode^\circ\else\textdegree\fi{}C is nearly twice as large as the current at room temperature. As the temperature however is raised from 600\ifmmode^\circ\else\textdegree\fi{}C to 750\ifmmode^\circ\else\textdegree\fi{}C the increase in photoelectric current for steady illumination is 100-fold while for interrupted illumination it is only 17-fold. The smaller increase in the latter case may be explained in view of the elimination of all sluggish currents produced by light in some secondary way. While the two curves for steady and interrupted illumination are similar in appearance, the latter has a maximum at 560\ifmmode^\circ\else\textdegree\fi{}C which resembles very much a resonance peak.The increase or decrease in photoelectric sensitivity with temperature for composite surfaces consisting of barium and oxygen on platinum may be explained by the diffusion of barium or oxygen on the surface. The increase in saturation voltage of the potential-current curves with temperature may likewise be explained by an increase in the contact potential brought about by a change in the work function.