Total Impurity Content Research Articles

Anomalous diffusion in nonmetals (origin and effects of internal electric fields)

When electrically active impurities or defects diffuse in a material with a band gap ≳ 10kT, the changing concentration of these ions may result in internal electric fields which will significantly alter the diffusion process, and result in gross misinterpretation of diffusion experiments. Equations are derived which describe the diffusion of interstitial impurities in such a system, and which take proper account of compensating immobile impurities. A technique for numerical solution of these equations is described and the results of calculations for the case of H+ in TiO2 are presented. These calculations predict a large enhancement of the over−all diffusion rate and a very distinctive concentration profile. The latter is of critical importance in identifying this effect, since our calculations show that a measurement of total impurity content vs time will appear to agree fairly well with Fick’s law; however, the apparent diffusion coefficient obtained in this way can be in error by a factor of 100 or more. Experimental results for the H+−TiO2 system are presented, which agree quantitatively with our calculations.

Shallow-donor ionization energies in the II-VI compounds

Hall data on ZnSe, CdS, CdSe, and CdTe have been analyzed to determine the shallow-donor ionization energies ${E}_{d}$ and the total donor and acceptor impurity concentrations ${N}_{d}$ and ${N}_{a}$, respectively. Lowering of the ${E}_{d}$ values owing to impurity concentration effects are noted even in the purest samples. When the data are analyzed using linear plots of ${E}_{d}$ vs ${({N}_{{d}^{+}})}^{\frac{1}{3}}$, the resulting values of the ionization energy for infinite dilution are in good agreement with optical data and with the hydrogenic model.

The growth and perfection of phenanthrene single crystals: 1. Purification and single crystal growth

A study has been made of the purification of phenanthrene, and the growth of large crystals from the melt by the Bridgman technique and their perfection. Of eight major impurities in the starting material four have segregation coefficients, k < 0.6 and segregate efficiently during zone refining. The remaining four which were identified as anthracene ( k = 1.6), flourene ( k = 0.95), dibenzothiophene ( k = 0.8) and dihydrophenanthrene ( k = 0.8) are inefficiently removed. When these are removed chemically, subsequent zone refining yields material in the centre part of the refined ingot of total impurity content ⪡ 1 ppm. When this material is used for crystal growth by the Bridgman technique, crystals of excellent quality can be produced. The influence of impurities on the growth process is outlined.

Lithium hydride single crystals

A single crystal of lithium hydride (LiH) was grown by a variation of the Stockbarger technique 1–3 used for lithium fluoride (LiF). The single crystal was cleaved into useful sizes, its purity investigated, and its microstructure studied by optical microscopy and scanning-electron microscopy (SEM). The single crystals were found to contain less than 0.18 wt% oxygen and a total impurity content of 0.20 wt%. Cleavage was apparent along two planes intersecting at 45° and 90° angles. This is believed to be caused by cleavage along {100} planes (primary cleavage) and along {110} planes (secondary cleavage). Slip planes, twins, kink bands, and cleavage-step patterns were observed in lithium hydride crystals.

Epitaxial growth of very pure NiO single crystals

This paper describes the procedure for growing epitaxial single crystals of NiO with a minimum content of impurities. Particular attention is paid to details of the starting material as well as to the cleaning and handling of the apparatus. Crystals with a total cation impurity content of less than 20 ppm are now grown routinely.

Model for Locally Compensated Cubic Sites of Tripositive Rare Earth Ions in Fluorite Crystals

The existence of a separate cubic phase RF3:3(MeF2), which provides a cubic symmetry site with local charge compensation for a tripositive rare earth ion in a fluorite lattice, is examined in order to explain the increase in cubic sites with increasing rare earth impurity concentration. An expression for the minimum concentration at which such a phase would be stable is presented. For the case of SrF2:Ge3+ or SrF2: (Gd3+, Ce3+) a calculation estimates that such a phase could form at 0.36% total rare earth impurity concentration.

Production and crystal growth of isotopic rare-earth metals

Large single crystals of high-purity rare-earth metal isotopes 154Sm, 163Dy, and 170Er were obtained by grain growth during high-temperature annealing. Isotopic separation produced about 60 g each of the isotopes in metal oxide form. The isotopic enrichment was 99.5, 96.9, and 96.1% for the 154Sm, 163Dy, and 170Er, respectively. The metal was formed by high-temperature reduction and distillation. Collection efficiency was about 92%, and the total impurity content was <500 ppm by weight. The metal was cast into a button in a water-cooled copper mold using a high-purity, arc-melting facility. Melt losses were about 50 mg for 163Dy and 170Er and 200 mg for 154Sm. The buttons were sealed in argon-filled tantalum cans, heated by induction, and annealed for 96 h at 917, 1375, and 1450 °C for the Sm, Dy, and Er, respectively. Any metal lost from the button was recoverable from the tantalum can interior. The resultant specimens had five to eight single crystal grains. The largest, 163Dy, was about 2.5 × 2.5 × 0.5 cm. All buttons had one or two grains large enough for neutron diffraction experiments. The substructure misorientation was <0.3° as measured by neutron diffraction.

Ordinary Magnetoresistance of Heavily Doped Semiconductors at Low Temperatures in a Weak Magnetic Field

AbstractThe ordinary longitudinal and transverse magnetoresistance of electrons at low temperatures in a weak magnetic field is calculated from a simplified model of heavily doped semiconductors. The treatment which is based on the Green's function methods using Feynman‐path integral formulation, is applied to disordered systems and developed already in five previous papers. Two important cases are discussed within the present model. In the first one, an ordinary n‐type semiconductor is considered, assuming that the carrier concentration is equal to the donor concentration; in the second case, the carrier concentration rises as the third root of the total concentration of impurities.

Electrolytic preparation of high-purity chromium

The preparation of high-purity chromium from commercial, aqueous electrolytic chromium has been investigated. Factors influencing the refining process, such as cell design, electrolyte components, chromium concentration in electrolytes, and cell operating parameters, have been studied. An improved molten-salt electrolytic refining process has been developed for the preparation of high-purity chromium with total impurity contents of less than 70 ppm, using a KClLiClCrCl 2 electrolyte at 520 °C. High-purity chromium deposits ranging from 0.2–0.3 kg have been routinely prepared with initial cathode current densities of 40–210 A/ft 2 with average current efficiency of 96%. The chromium recovery in the process was 99%.

Weak Magnetic Field Dependence of the Thermoelectric Effect in Heavily Doped Semiconductors at Low Temperatures

AbstractThe field dependence of the thermoelectric effect is studied, using a simplified model of heavily doped semiconductors, by Green's function methods and the Feynman‐path integral formulation applied to disordered systems, already developed in five previous papers. Two important cases are discussed. In the first, an ordinary n‐type semiconductor is considered, assuming that the carrier concentration is equal to the donor one; in the second, the carrier concentration is rised as the third root of the total concentration of impurities.

Isotherme galvanomagnetische Transport- koeffizienten bei Streuung quasifreier Ladungsträger an Dipolen / Isothermic galvanomagnetic transport coefficients of quasi-free carriers scattered, at dipole centres

Abstract It has been observed that in several semiconductors an in-crease in total impurity concentration leads to an increase in the low-temperature mobility due to pairing effects. The trans port coefficients rʜ, rQ' and rQ necessary for the analysis of the Hall effect and the magnetoresistance, in the event of di-pole scattering, are derived for the cases k a ≫ 3 and k a ≪ 3 (k = wave number of the quasi-free charge carrier, a = dipole length). For k a ≫ 3 the coefficients are dependent on tempe-rature, effective mass, and dipole length, whereas for k a ≪ 3 they are constant

High-temperature creep of polycrystalline chromium

A study has been conducted to determine the high-temperature creep properties of high-purity, polycrystalline chromium. Iodide chromium was consolidated into 100 g buttons by arc melting and fabricated to sheet by rolling. Test specimens machined from the wrought sheet were annealed in palladium-purified hydrogen in order to produce stable grain structures for testing. Chemical analysis indicated a total interstitial impurity content of < 100 ppm. Step load creep tests were conducted over the temperature range 0.51–0.78 T m . Results showed that creep rates can be represented by the general creep equation g3 D = k σ n E where n, for chromium, is equal to 4.3. This correlation, which uses the previously determined activation energy of 73.2 kcal/mol for self diffusion in chromium, and observations of surface slip lines and dislocation substructures strongly suggest that a diffusion controlled, dislocation climb mechanism is operative in the creep of chromium over the temperature range investigated.

Relationship between Carrier and Impurity Concentrations in Vapor-Epitaxial n-GaAs Crystals

The relationship between the room-temperature carrier concentration n0 and the total impurity concentration NI is investigated in order to obtain some information on the natures of the residual impurities and the native crystal defects in epitaxial n-GaAs layers grown by the Ga/ASCl3/H2 method. The n0vsNI curves observed are compared with that calculated from the Rytova-Fistul' theory which considers thermo-dynamically the amphoteric impurity distribution in compound semiconductors. Both are in good agreement if some unknown parameters are reasonably adjusted. Preliminary check is also made on the effect of the arsenic partial pressure on the values of these parameters. From these results it is supposed that the electrically-active centers in epitaxial n-GaAs layers mainly come from amphoteric impurities and the arsenic vacancies also play a significant role as compensating acceptors.

The significance of dynamic strain aging in titanium

Tensile tests were conducted at a 0.02 in./min cross head speed on high purity and commercial purity titanium specimens between 77 and 1026°K to determine if dynamic strain aging is of practical importance in titanium. The data show that reducing the total impurity concentration from approximately 3300 to 300 ppm has a very strong effect on the strain aging characteristics of titanium between about 500 and 900°K. It removes the rate dependent work hardening phenomena, lowers the magnitude of the strain aging component of the yield stress by a factor of probably four, eliminates the Portevin-Le Chatelier effect, largely suppresses the “blue brittle” phenomena and greatly reduces the strong and variable effect of temperature and strain rate upon the shapes of the stress-strain curves. While the discontinuous yield phenomena in titanium are weaker than those in steel, the rate dependent work hardening in these two metals are almost comparable.

Magnetic susceptibility of heavily doped semiconductors in a weak magnetic field

AbstractBy Green function methods applied to disordered systems, the magnetic susceptibility of heavily doped semiconductors in a weak magnetic field is investigated. A simplified model is used in which the impurity potential is an attractive screened Coulomb potential and quantum exchange‐correlation effects can be neglected. Two important cases arc discussed with the present model. In the first one, we consider an ordinary n‐type semiconductor, assuming that the carrier concentration is equal to the donor one; in the second case, the carrier concentration rises as the cube root of the total concentration of impurities.

The Reactivity of Vanadium and Some Vanadium Alloys with Low-Pressure Oxygen and Nitrogen

The reactivity of pure vanadium (&amp;lt;350 ppm total impurity content) and the alloys vanadium–10.5-wt% chromium, vanadium–9.5-wt% molybdenum, and VANSTAR-7 (V–9 wt% Cr–3 wt% Fe–l.3 wt% Zr–0.05-wt%) with oxygen and nitrogen at low partial pressures in the temperature range of 600–800 °C was determined utilizing a hot-wall ultrahigh vacuum microbalance system. The rate of reactivity was determined from continuous weight-gain measurements as a function of both temperature and pressure, for oxygen partial pressures of 10−5 to 10−7 Torr and nitrogen partial pressures of 10−4 to 10−6 Torr. Linear kinetics were observed, indicative of a process controlled by gas transport to the metal surface. The sticking probability for oxygen of 0.185±0.040 was found to be independent of temperature, pressure, and alloying additions. The rate of reaction with nitrogen was a function of alloying additions, with VANSTAR-7 and V-10.5Cr exhibiting the lowest reactivity. The sticking probabilities were compared with those reported in the literature. These results indicate that the sticking probabilities of low-pressure gases on refractory metals are not well defined and seem to be a function of the particular experimental apparatus utilized.

Electrical Properties of Single Crystals, Bicrystals, and Polycrystals of MgO

The high oxygen pressure conductivity of high‐purity single‐crystal magnesium oxide at high temperatures varied as the 1/4 power of the oxygen partial pressure with a 3 eV activation energy; the low pressure conductivity varied as the ‐1/6 power of the pressure with an activation energy of 4 eV. The predominant defects proposed are (1) holes and singly ionized Mg vacancies at high pressure and (2) electrons and doubly ionized oxygen vacancies at low pressures. The effect of impurities was noted. Changes in the grain boundary conductance relative to the crystal conductance of MgO were observed as a function of temperature and pressure, but not as a function of grain boundary orientation, transport direction, or total impurity content. Increased high pressure ionic conductivity in polycrystalline material offered possible evidence for increased grain boundary diffusion.

Preparation of high purity thorium and thorium single crystals

Ultra-high purity thorium has been prepared using electrotransport as the refining method. This metal has a total impurity content of less than 50 p.p.m. and a resistance ratio of greater than 1000. A procedure was also developed by which single crystals of the high purity thorium could be prepared by repeated heating of the specimen through its alpha to beta phase transformation followed by prolonged heating just below the transformation temperature.

The Use of the Direct Reading Spectrometer with dc Arc Excitation and Total Sample Combustion in the Analysis of High Purity Materials

When a dc arc is used for trace analysis on a spectrometer, calibration curves with a pronounced toe due to spectral background are found. For analysis of high purity materials it is most important that linear relationships should be established to ultra-trace levels when total impurity content is specified within the ppm level. A solution to this problem is outlined and the linear relationships obtained for copper, bismuth, and cadmium in high purity silver to 0.1 ppm are shown. An electronic circuit is described that enables these background corrections to be made.

Electron-Beam Float Zone and Vacuum Purification of Vanadium

Electron-beam float zone melting vanadium three passes at 3.9 in./h in 4×10−10 Torr vacuum resulted in resistance ratios of about 700 and a total impurity content of 50 wt ppm. The zoning reduced both carbon and oxygen content. However, while carbon removal was always accompanied by a lowering of the oxygen content, the reverse was not true. The removal of oxygen was probably accomplished by the formation and volatilization of VO. The mechanism for carbon removal was not clearly resolved. Nitrogen was not appreciably affected by electron-beam float zone melting. All metallic impurities, with the exception of Ta and W and possibly Si, were effectively volatilized. No evidence was found for zone refining action on any impurity. Vacuum outgassing further reduced the carbon and oxygen level. However, the oxygen removal was far more effective than the carbon. The nitrogen content increased, particularly on the specimens that lost about 40% in weight due to vanadium evaporation. However, the effect of vacuum outgassing was less on the highest purity as-zoned specimens. The same purification mechanisms apparently operated.