Showing papers by "National Institute of Standards and Technology published in 1971"

Inelastic Collisions of Fast Charged Particles with Atoms and Molecules-The Bethe Theory Revisited

Abstract: Since the appearance of the title paper, a number of new developments have occurred which need to be included in that body of material. We present additional remarks and clarifications which supplement and update numerous aspects of the Bethe theory discussed in the earlier paper. We also bring the bibliography up to date. Plasma stopping power, the ${z}^{3}$ effect, and stopping power for particles at extreme relativistic energies are among the new topics included. We make several comments on Fano's earlier review article, Ann. Rev. Nucl. Sci. 13, 1 (1963).

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Abstract: Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al2O3. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.

Correlation Factors for Diffusion in Nondilute Alloys

Abstract: Correlation factors for diffusion in binary and multicomponent alloys are calculated for a random-alloy model with diffusion by a vacancy mechanism. This model, which should apply best for nondilute alloys, assumes that atoms and vacancies are randomly distributed and that suitable average values can be used to represent the actual atom and vacancy jump frequencies in the crystal. In alloys, both atoms and vacancies follow correlated walks. Also, the atom correlation factors are influenced by the nonrandom motion of the vacancies. Thus, in order to treat correlation effects in concentrated alloys properly, one must consider not only the correlation factors ${f}_{i}$ for diffusion of atoms but also the correlation factor ${f}_{v}$ for diffusion of vacancies. In specific calculations, one also must find the partial correlation factors ${f}_{v}^{i}$ for diffusion of vacancies by exchange with atoms of the particular species $i$. Analytic expressions for all of these correlation factors are calculated. These equations can be expressed directly in terms of the measureable tracer-diffusion-coefficient ratios $\frac{{D}_{i}^{*}}{{D}_{k}^{*}}$ with no unknown jump frequencies appearing. The calculations also yield a forbidden region in the plot of diffusion-coefficient ratio as a function of alloy composition, with correlation factors going to zero at the boundary of this region. Specific applications to binary alloys are discussed.

Raising of Discrete Levels into the Far Continuum

Abstract: Broad peaks observed in the photoabsorption spectra of rare earths \ensuremath{\sim}10-20 eV above the $4d$ edge are attributed to transitions $4{d}^{10}4{f}^{N}\ensuremath{\rightarrow}4{d}^{9}4{f}^{N+1}$. Exchange interaction splits the $4{d}^{9}4{f}^{N+1}$ configuration and raises some multiplets by \ensuremath{\sim}20 eV. Autoionization to $4{d}^{9}\ensuremath{-}4{f}^{N}\ensuremath{\epsilon}f$ broadens the high levels. The interpretation extends to $p\ensuremath{\rightarrow}d$ processes observed in the absorption of synchrotron light by transition elements.

The temperature dependence of stacking fault energy in Fe-Cr-Ni alloys

Abstract: The variation of intrinsic stacking fault energy (γi) in two austenitic Fe-Cr-Ni alloys has been determined from dislocation node measurements over the range 25‡ to 325‡C by means of high temperature transmission electron microscopy In both alloys γi increases with temperature Both reversible and irreversible effects have been observed in cyclic heating-cooling experiments After the first heating to elevated temperature the irreversible component is removed and thereafter cyclic annealing produces essentially reversible changes in γi The large reversible changes are best understood in terms of the variation in the stability of austenite with temperature The smaller irreversible effect appears to arise from the formation of substitutional solute atmospheres around partial dislocations at elevated temperature

Some Properties of the Range in Samples from Tukey's Symmetric Lambda Distributions

Abstract: Tukey introduced a family of random variables defined by the transformation Z = [Uλ - (1 - U)γ]/λ where U is uniformly distributed on [0, 1]. Some of its properties are described with emphasis on properties of the sample range. The rectangular and logistic distributions are members of this family and distributions corresponding to certain values of λ give useful approximations to the normal and t distributions. Closed form expressions are given for the expectation and coefficient of variation of the range and numerical values are computed for n = 2(1)6(2)12, 15, 20 for several values of λ. It is observed that Plackett's upper bound on the expectation of the range for samples of size n is attained for a λ distribution with λ = n − 1.

High-Speed (Subsecond) Measurement of Heat Capacity, Electrical Resistivity, and Thermal Radiation Properties of Molybdenum in the Range 1900 to 2800 K.

Abstract: A technique is described for the high-speed measurement of heat capacity, electrical resistivity, hemispherical total and normal spectral emittances of electrical conductors at high temperatures (above 1900 K) with millisecond resolution. Duration of an individual experiment, in which the specimen is heated from room temperature to close to its melting point, is less than one second. Temperature measurements are made with a high-speed photoelectric pyrometer. Quantities are recorded by a high-speed digital data acquisition system which has a resolution of approximately one part in 8000. Time resolution of the entire system is 0.4 ms. Results on the above properties of molybdenum in the temperature range 1900 to 2800 K are reported and are compared with those in the literature. Estimated inaccuracy of measured properties in the above temperature range is: 2 to 3 percent for heat capacity, 0.5 percent for electrical resistivity, 3 percent for hemispherical total emittance and 2 percent for normal spectral emittance.

Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet

Abstract: The measurements have been conducted in the spectral range from 600 to 2000 A. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 A. From the photoionization curve the first adiabatic ionization potential was found to be 10.87 plus or minus 0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states.

Enhancement of absorption spectra by dye-laser quenching

Abstract: Absorption spectra enhancement by organic dye laser quenching, considering applications for absorbing species spectroscopic detection

Observation of Metal-Vacuum-Metal Tunneling, Field Emission, and the Transition Region

Abstract: We report what we believe are the first observations of metal-vacuum-metal tunneling. A field emitter is brought close to a metal surface and the current-voltage characteristic is measured in three regions: the Fowler-Nordheim region, the intermediate region, and-and the metal-vacuum-metal region.

Thermodynamics, Chemical Reactions and Molecular Biology

Abstract: An extended statement of chemical equilibrium calls for consideration of the integral ∫T0 ΔCP dT, in addition to T ∫T0 ΔCp/T dT (= T ΔST0) and ΔHT0—the classical terms of the equation −RTlnK = ΔHT0 = T ΔST0. The integral ∫T0 ΔCP dT is not negligible for macromolecules and particularly biopolymers, and its direct experimental determination at all temperatures down to 0 K is therefore indispensable for thermodynamic understanding of the objects of molecular biology.

DyP O 4 : A Three-Dimensional Ising Antiferromagnet

Abstract: The magnetic susceptibility, heat capacity, and optical-absorption spectrum of Dy P${\mathrm{O}}_{4}$ have been measured as a function of temperature and magnetic field. The optical-absorption spectrum indicates that the magnetic interactions in Dy P${\mathrm{O}}_{4}$ have the form of the Ising interaction and occur primarily between nearest neighbors. The magnetic-susceptibility and heat-capacity measurements have been compared with exact series expansions for a diamond lattice assuming an Ising system with nearest-neighbor interactions. The theoretical calculations are in agreement with the measurements. The nature of the divergence of the heat capacity at the critical point is examined. Our data do not permit us to distinguish between the applicability of the logarithmic and power-law divergencies since they are indistinguishable in the region of our measurements. The temperature dependence of the critical field for the metamagnetic phase transition has also been determined and compared with the calculated value for the critical field at 0 K obtained from the spectroscopic measurements.

Unified Theory Calculations of Stark Broadened Hydrogen Lines Including Lower State Interactions

Abstract: Report on calculations of hydrogen Stark broadening on the basis of the unified classical path theory which has been extended to include lower state interactions in the final line profile. A detailed comparison with experiments in the density range 10-13-10-17 cm-3 is given.

Approach to Alkali-Metal Chemisorption within the Anderson Model

Abstract: A theory of chemisorption relevant to alkali atoms on metal surfaces is presented. The virtual-impurity-state problem is modeled in the manner presented by Kj\"ollerstr\"om, Scalapino, and Schrieffer for solving the Anderson impurity problem in the low-density approximation (LDA). It is assumed that the binding or chemisorption energy of the alkali metal is composed of two parts, a metallic and an ionic component. Since considerable charge transfer from the alkali atom to the metal occurs, the LDA is appropriate for describing the metallic part of the binding. Another consequence of the large charge transfer is that the major portion of the binding results from an ionic type of bond between the partially charged alkali ion on the surface and a polarization or screening charge inside the metal. The results of the theoretical calculations indicate that binding energies for alkali atoms adsorbed on clean single-crystal faces of metals generally fall with in the range of 1.5-2.5 eV, in accord with available experimental data on metals such as W, Mo, Ta, and Ni.

Dilute Solution Theory of Polymer Crystal Growth: A Kinetic Theory of Chain Folding

Abstract: A kinetic theory of polymer crystallization from dilute solution is formulated for linear chain molecules of finite molecular weight (monodisperse). Two models of crystal growth are considered; both are essentially “regular” chain folding type models. Formulas for the crystal growth rates are derived as a function of the fundamental rate constants associated with the various states of molecular crystallization. These rate constants are evaluated as a function of polymer concentration, molecular weight, crystallization temperature, and crystal thickness. Consideration of finite molecular weight molecules requires an understanding of how these molecules are incorporated into the crystal and what happens to chain ends. Attention is focused on these problems and a description of how “cilia” are formed in polymer crystals is given. A remarkable aspect of cilia formation is that the uncrystallized portion of a chain molecule which dangles in the solution can participate in nucleating a new growth strip (fold pl...

The searchlight problem with isotropic scattering

Abstract: A solution to Chandrasekhar's searchlight problem with isotropic scattering is presented for semi-infinite and finite geometries. The scattering and transmission functions for the Fourier components of intensity are given by expressions analogous to those of the plane-parallel case, except that the physical domain of the variable ζ, which replaces the usual direction cosine, is now the circular region |ζ 1 2 | ≤ 1 2 in the complex plane, and also that the corresponding H-, X-, and Y-functions now depend on a scalar wave number. Nonlinear integral equations for H, X and Y are derived, and numerical methods for computing these functions are given by means of the kernel approximation method.

The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Abstract: The crystal structure of Ca5(PO4)2SiO4 (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) A,b=15.508 (2) A andc=10.132 (1) A at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm−3. The crystal contains about 2.5% V2O5 as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO4 3−, SiO4 4− and possibly VO4 3− occurs among the anion sites. The structure has some relationship to that of Ca5(PO4)3OH, the predominant inorganic phase in the human body, but suggests that the Ca5(PO4)3OH type structure may not be stable without some of the OH positions being filled. Ca5(PO4)2SiO4 is more closely related to K3Na(SO4)2 (glaserite) if it is considered that there are systematic cation vacancies in Ca5(PO4)2SiO4. This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca2SiO4 and Ca3(PO4)2 and between Ca3(PO4)2 and CaNaPO4.

Three-Fluid Hydrodynamical Model of Nuclei

Abstract: A three-fluid model of nuclei is introduced, the three fluids being the protons, the neutrons of the same orbitals as protons, and the excess neutrons, to account for the fact that the excess neutrons interact less strongly with the protons than do the neutrons which occupy the same space-spin states as the protons. Calculations of proton and neutron density distributions, of isotope shifts, and of isospin impurities have been carried out. The giant-dipole phenomenon is also studied in the present model. It is found that considerable improvement is achieved in the results for the proton density distribution and for the isospin impurities as compared with the two-fluid model. The other results are found to be consistent with previous calculations.