Control Of Vapor Pressure Research Articles

Stoichiometry control for growth of III–V crystals

The most important factor for compound semiconductors such as the III–V and II–VI compounds is the deviation from the stoichiometric composition, because the deviation corresponds to imperfections in the crystals which cause large changes in characteristics. The specific vapor pressure at which the crystal becomes stoichiometric was determined in the cases of heat-treatment and of crystal growth using LPE and melt growth techniques. The temperature difference method under controlled vapor pressure (TDM-CVP) is the most suitable growth method which can control the deviation from the stoichiometric composition by applied vapor pressure, even without direct contact to the solid. The optimum vapor pressures to produce nearly perfect crystals with very low dislocation densities were found as a function of temperature and were on the same straight line. The most important point defect for GaAs at high arsenic vapor pressure was found to be As interstitial atoms which play a very important role in inducing many kinds of deep levels, not only by anti-site arsenic atoms. The use of liquid encapsulation at the rotating seed shaft rather than at the melt surface has been introduced for Czochralski pulling. Nearly perfect GaAs crystals with a diameter of 15 cm were grown with very brilliant surfaces. Deep levels, in concentrations as low as 10 15 cm -3, are decreased with the application of optimum vapor pressure, and somewhat reversed with an increase of the impurity concentration. The theory for the mechanism of vapor pressure control is described by the concept that the chemical potentials of As in the three phases, vapor liquid and solid, are set equal to each other.

Adsorption of Water by Homoionic Exchange Forms of Wyoming Montmorillonite (SWy-1)

AbstractAdsorption isotherms for water on homoionic (Ca2+, Li+, Na+, and K+) exchanged forms of Crook County, Wyoming, montmorillonite (CMS Source Clay SWy-1) were measured between 25° and 70°C using a vacuum microbalance having automated control of water vapor pressure. From these adsorption data, integral enthalpies and entropies of adsorption were calculated. Both quantities were negative, but decreased in magnitude with increasing amounts of adsorbed water. For all four cationic forms of the clay, the amount of initial water adsorption at 25°C and at low relative humidities was sensitive to the sample temperature during prior evacuation of water, less water being adsorbed by samples evacuated at 100°C compared with samples evacuated at 25°. For Ca- and Na-montmorillonite, these changes were reversible after several subsequent desorption/adsorption cycles, but recovery was not observed for the Li-SWy-1 clay, probably because of migration of Li+ into the aluminosilicate structure.

Synthesis, Magnetic, Electrical and Mössbauer Investigations on Complex Semiconductors

ABSTRACTThe system Ba-Fe-S contains a phase Ba1+xFe2S4 whose structure consists of infinite chains of FeS4 edge sharing tetrahedra with Fe-Fe distances of 2.7Å. The chains are 6 Å apart and Ba2+ fills the space between the chains. Each value of x designates a distinct phase and such structures have been labeled “infinitely adaptive.” The Ba1+xFe2S4 compounds are anisotropic semiconductors. The room temperature electrical conductivity for the x = 0 and x = 0.086 phases changes by nearly 6 orders of magnitude as increasing x populates the conduction band. The preparation of these materials requires very careful control of sulfur vapor pressure in closed ampules at elevated temperatures. A similar effect is observed in rare earth sulfides with the Th3P4 structure. Intrinsic La2S3 and BaLa2S4 are predicted to be high resistivity semiconductors. As the composition changes to a stoichiometry such as Ba0.8La2.2S4 the behavior becomes semi-metallic. Control of the sulfur vapor pressure will be a critical parameter for designing physical parameters into these compounds.

Bridgman‐Type Apparatus for the Study of Growth‐Property Relationships: Arsenic Vapor Pressure ‐ GaAs Property Relationship

A precision Bridgman‐type apparatus is described which was designed and constructed for the investigation of relationships between crystal growth parameters and the properties of crystals. Several key features of the system are highlighted, such as the use of a heat pipe for precise arsenic vapor pressure control and seeding without the presence of a viewing window. Pertinent growth parameters, such as arsenic source temperature, thermal gradients in the growing crystal and in the melt, and the macroscopic growth velocity can be independently controlled. During operation, thermal stability better than ±0.02°C is realized; thermal gradients can be varied up to 30°C/cm in the crystal region and up to 20°C/cm in the melt region; the macroscopic growth velocity can be varied from 50 μm/hr to 6.0 cm/hr. It was found that the density of dislocations depends critically on As partial pressure; and essentially dislocation‐free, undoped, crystals were grown under As pressure precisely controlled by an As source maintained at 617°C. The free carrier concentration varied with As pressure variations. This variation in free carrier concentration was found to be associated with variations in the compensation ratio rather than with standard segregation phenomena.

Convectively mixed humidity chamber

A humidity chamber is described, based on the principle of thermodynamically regulated (vapor pressure) control, in which the attainment of thermodynamic equilibrium is enhanced by gentle convective stirring. It may be used as a chamber which maintains constant dew point while allowing variation of the temperature of the test section, or the test section may be kept at constant temperature while the dew point temperature is varied. Dew point temperatures may be produced with a certainty of 0.3 and 0.1 °C at −20 °C and +30 °C, respectively; calculated relative humidities have an uncertainty of 0.1%–0.6% relative humidity for low to high humidities, which correspond to errors of 4% and 1%, respectively. The chamber has been operated down to ‐200 °C.

A new method of sulfur vapor pressure control and its application to the V-S system

A new apparatus for obtaining partial pressure of sulfur was devised by using liquid sulfur and carrier gas of nitrogen. This successfully works in the range of ∼10 −4 to ∼1 atm of Ps 2 at any temperature higher than about 400°C up to about 1000°C. As an application of it, the equilibrium study of V-S system was done at 800°C. Two phases V 3S 4 and V 5S 8, each having homogeneity range, were found while no existence of the V 2S 3 phase could be detected thermodynamically and X-ray crystallogrpahically at this temperature. The standard free energy of following reaction, VS 4 3 + 2 15 S 2 = VS 8 5 , was calculated by considering the transition-Ps 2 and the activities a VS 8 5 and a VS 4 3 · ΔG° (1073K) = −1150 cal·mole −1was obtained with an estimated uncertainty of ± 100 cal to the above reaction.

The Influence of Added H 2 O and D 2 O on Anoxic Radiation Sensitivity in Bacterial Spores

With careful control of water vapor pressure for equilibrating dried spores of Bacillus megaterium, changes were measured in the ${\rm O}_{2}\text{-independent}$ component ( $k_{{\rm I}}$ ) of radiation damage over the narrow range in which water increase causes a rise in the inactivation constant. The measures were made for both H2 O and D2 O as added water. For added H2 O, $k_{{\rm I}}$ begins to increase at a water content below and is complete at a content above that which represents a monolayer in the cell (1.2 molecules ${\rm H}_{2}{\rm O}/{\rm DNA}$ phosphate); for D2 O the behavior is the same but quantitatively different (1.3 molecules ${\rm D}_{2}{\rm O}/\text{phosphate}$ ). This suggests that sensitization is caused by water radiolysis in a water shell immediately external to the target molecule, and the fact that in D2 O the spores are slightly more sensitive than in H2 O points to the hydroxy...

A method for control of water vapor pressure and its effect on metabolism and body temperature in Mus musculus

1. 1. A simple method for controlling water vapor pressures (WVPs) in an open-flow system is described. 2. 2. High WVPs (30 mm Hg) alter the normla homeothermic response to ambient temperature variation in laboratory mice such that metabolic rate becomes independent of ambient temperature, particularly at ambient temperatures above the thermoneutral zone. 3. 3. WVPs of 10 and 20 mm Hg do not affect the normal, metabolism-ambient temperature relationship.

Control of Mercury Vapor Pressure of Fluorescent Lamps by Indium-Mercury Amalgam

Characteristics of fluorescent lamps are influenced remarkably by mercury vapor pressure. Therefore. it is most important to control mercury vapor pressure.This paper deals with a new method and theory of vapor pressure control. Experiments are carried out with indium-mercury amalgam.As the results, mercury vapor Pressure is controlled mainly by electron current flow in the amalgam. And mercury vapor pressure of steady state lamps is decided by the correlation of amalgam and the coolest spot in the bulb wall. Light output-vs-ambient temperature curve becomes broader by the combination of this method and temperature compensating elements.

Capillary Membrane Hydrogen-Oxygen Fuel Cell System for Space Vehicle Application

This paper describes the development of a simple, effective method of electrolyte concentration control for the Allis-Chalmers Capillary Membrane fuel cell. The method, designated as Vapor Pressure Control, operates on the change in electrolyte vapor pressure which occurs over an operable range of electrolyte concentrations. Stability of cell operation with the method is shown theoretically and is verified by tests on two four-cell modules for durations up to 113 hours.