Showing papers in "Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry in 1963"

Measurement of the Thickness and Refractive Index of Very Thin Films and the Optical Properties of Surfaces by Ellipsometry.

Abstract: The use of the ellipsometer for the measurement of the thickness and refractive index of very thin films is reviewed. The Poincare sphere representation of the state of polariza­ tion of light is developed and used to describe the reflection process. Details of the operation of the ellipsometer are examined critically. A computational method is presented by which the thickness of a film of known refractive index on a reflecting substrate of known optical constants may be calculated directly from the ellipsometer readings. A method for comput­ ing both the refractive index and thickness of an unknown film is also developed. These methods have been applied to the determination of the thickness of an adsorbed water layer on chromium ferrotype plates and on gold surfaces. In the former case the thickness was 23 to 27 A, and in the latter was 2 to 5 A. The measurement of the thickness and refractive index of barium fluoride films evaporated on chromium ferrotype surfaces is used as an illustration of the simultaneous determination of these two quantities.

Melting Temperature and Change of Lamellar Thickness with Time for Bulk Polyethylene.

Abstract: The melting temperature of linear polyethylene has been obtained as a function of the time and temperature of crystallization. Recrystallization was minimized by a special melting procedure. By interpreting the melting points as characteristic of a given lamellar thickness, it was found that the thickness of crystals of appreciable age increased linearly with the logarithm of their time of existence. The lowest melting (i.e., thinnest) lamellae in a given specimen may be assumed to have either existed for only a short period of time, or to have been impeded in their growth in the chain direction, and they were found to have an estimated thickness close to that predicted by recent kinetic theories of polymer crystal growth with chain folding.

Dynamic Compressibility of Poly (Vinyl Acetate) and Its Relation to Free Volume.

Abstract: The complex bulk compliance (dynamic compressibility) of a commercial sample of poly (vinyl acetate), AYAT, was measured at frequencies from 50 to 1,000 cycles per second, temperatures from 0 to 100 °C, and static hydrostatic stresses from 0 to 981 bars (gage pressure) using an alternating hydrostatic stress generated and detected by piezoelectric transducers mounted in an essentially noncompliant cavity with dimensions small in comparison to a wavelength. The above temperature range was more than sufficient to cover the dispersion region in which an inflection in the storage compliance and a maximum in the loss compliance were observed. The data were reduced to functions of reduced variables using the WLF Equations extended to include static pressure with the "universal" WLF Constants and dT/dP = 0.020 °C/bar. The difference in limiting compliances at zero and infinite frequencies was larger than that predicted from the dT/dP shift using the free volume concept. A discussion is presented on possible processes that might contribute to an excessive value between limiting compliances.

Analysis of the Spectrum of Neutral Atomic Bromine (Br I)

Abstract: The spectrum of the neutral bromine atom, Br I, has been newly investigated by using electrodeless discharge tubes as light sources. The observations have led to a list of wavelengths and estimated intensities for 1253 spectral lines in the range 1067 to 24100 A. The number of known energy levels has been increased to 123 even and 128 odd levels, as compared with the 27 even and 33 odd levels previously known. All predicted energy levels of the 4s 24p 4 ns, up, nd, nf electron configurations from 0 to ~93250 K have been discovered. The observations in the vacuum ultraviolet establish that the positions of all the levels lying above those of the ground configuration as given in the compilation Atomic Energy Levels, Vol. II (1952) should be increased by 6.7 K. All but 26 faint lines of Br I have been classified. A total of 67 levels has been ascribed to the 4s 2 4p* nf configurations. It is demonstrated that the nf configurations exhibit almost pure pair coupling. The very regular (3P2)nf[5]°11/2 series yields for the principal ionization energy of Br I the value 95284.8 K.

Effect of Pressure and Temperature on the Refractive Indices of Benzene, Carbon Tetrachloride, and Water

Abstract: An interferometer for measuring change in index of refraction with pressure is described. Absolute indices of refraction are reported to five decimals for benzene, carbon tetrachloride, and water at pressures as high as 1100 bars over a small temperature range. The results of replicate measurements agree to within ±0.0001. Various equations relating index and specific volume show systematic deviations in all cases. At constant specific volume, the index of carbon tetrachloride increases with increasing temperature, while the index of water decreases with increasing temperature. The refractive index of benzene shows no effect due solely to temperature within the experimental error. Possible explanations for this behavior are discussed.

Pressure-density-temperature relations of fluid para hydrogen from 15 to 100 deg k at pressures to 350 atmospheres

Abstract: Experimental data are presented at closely spaced intervals of temperature and density. The range of experimental densities is from 0.064 to 2.8 times the critical density. There are presented, in addition, tables interpolated uniformly in arguments density and temperature, and also in pressure and temperature.

Thermodynamic properties of magnesium oxide and beryllium oxide from 298 to 1,200 °K

Abstract: As a step in developing new standards of high-temperature heat capacity and in determining accurate thermodynamic data for simple substances, the enthalpy (heat content) relative to 273 deg K, of high purity fused magnesium oxide, MgO, and of sintered beryllium oxide, BeO, was measured up to 1,173 deg K. A Bunsen ice calorimeter and the drop method were used. The two samples of BeO measured had surfaceto-volume ratios differing by a factor of 15 or 20, yet agreed with each other closely enough to preclude appreciable error attributable to the considerable surface area. The enthalpies found for MgO are several percent higher than most previously reported values. The values are represented within their uncertainty (estimated to average plus or minus 0.25%) by empirical equations. Values of enthalpy, heat capacity, entropy, and Gibbs free-energy function are tabulated from 298.15 to 1,200 deg K. (auth)

Wavelength Calibrations in the Far Infrared (30 to 1000 Microns).

Abstract: A discussion is presented of certain calibration procedures employed in the region 30 to 1000 microns. Calculated positions for the pure rotational absorption lines of the CO, HCN, and N2O molecules are given, and a map of the pure rotational absorption lines of the H2O molecule as recorded with a Perkin-Elmer model 301 spectrophotometer is shown.

Reactions of Polyfluorobenzenes With Nucleophilic Reagents.

Abstract: Nucleophilic reactions of hexafluorobenzene and related polyfluorobenzenes were studied in detail. Reaction of hexafluorobenzene with hydroxides, alcoholates, aqueous amines, and organolithium compounds led to the substitution of one or more fluorine atoms. The structures of the products were determined, using near infrared and nuclear magnetic resonance spectra. Fluorine is replaced more readily than chlorine, bromine, iodine, or other groups. In the majority of the products in which two of the fluorines in hexafluorobenzene were replaced, the substituting groups were para to each other. However, depending on the reagents other orientation effects were noted. The reaction mechanisms were a function of reagents and conditions. The most prevalent mechanism is presumably the displacement of a fluoride anion by another anion, probably via the formation of transition complexes of different lifetimes. However, simple ionization or attack by neutral species may occur under some conditions. The diazotization and oxidation of pentafluoroaniline were also investigated.

Thermodynamic Properties of Polyethylene Predicted from Paraffin Data.

Abstract: Thermodynamic data on the n-paraffins from n-C6H14 through n-C18H38 have been used to obtain values for the specific heat, entropy, enthalpy, and Gibbs free energy of a large, ideal CH2-chain crystal from 0 to 420 °K and of the liquid above 200 °K. Analytical expressions are given for the properties of the crystal and liquid above 200 °K. For the crystal, a modified Einstein function was used to adjust the melting temperature to 414.3 °K. Values between 975 and 1025 cal/mole for the heat of fusion were found to be the ones most consistent with the data. Comparison of the results with polyethylene data shows reasonable agreement.

Phase Equilibrium Relations in the Sc2O3-Ga2O3 System.

Abstract: The phase equilibrium diagram was determined for the Sc2O3-Ga2O3 system. A quenching furnace, wound with 60 percent Pt-40 percent Rh wire, was employed for experiments conducted at temperatures up to 1,800 °C. An induction furnace, having an iridium crucible susceptor, was used to obtain higher temperatures. Temperatures in the quenching furnace were measured with both an optical pyrometer and a 95 percent Pt-5 percent Rh versus 80 percent Pt-20 percent Rh thermocouple. The melting point of Ga2O3 was determined as 1,795 ±15 °C. Experiments at temperatures as high as 2,405 °C failed to melt Sc2O3. Two intermediate binary phases, a compound believed to be 6Sc2O3·5Ga2O3 and a solid solution occur in the system. The solid solution phase appears as a single phase in the region roughly defined by the compositional limits of 55 to 73 mole percent Ga2O3 at the solidus. The 6:5 compound, stable only at high temperatures, melts incongruently at 1,770 ±15 °C and decomposes below 1,700 ±15 °C. The compound appears to have orthorhombic symmetry with a=13.85 A, b= 9.80 A, and c=9.58 A. The indicated uncertainties in the melting points are a conservative estimate of the overall inaccuracies.

Cyclic Polyhydroxy Ketones. I. Oxidation Products of Hexahydroxybenzene (Benzenehexol).

Abstract: Reliable procedures are given for the preparation and purification of hexahydroxybenzene (benzenehexol), tetrahydroxy-p-benzoquinone, rhodizonic acid, triquinoyl (cyclohexanehexone), croconic acid, and leuconic acid (cyclopentanepentone). Certain derivatives and color tests, as well as infrared and ultraviolet spectra, are reported for their identification.

Thickness of Adsorbed Polystyrene Layers by Ellipsometry.

Abstract: The adsorption of polystyrene from cyclohexane below the theta temperature onto chrome ferrotype plate was studied by means of ellipsometry (polarization spectrometry). In this technique changes in the state of polarization of polarized light are measured upon reflection from a film-covered surface. The measurements were carried out in situ and permitted determination of the thickness and refractive index of the swollen polymer film at the solidsolution interface. A concentration range of 0.18 to 9.7 mg/ml was studied for polymer with a molecular weight of 76,000. The thickness of the adsorbed film increased with increasing solution concentration, reaching a plateau for most of the concentration range studied. The average thickness at this plateau was approximately 210 A. The adsorbed film was highly swollen, consisting of about 12 g/100 ml of polymer for most of the concentration range. The amount adsorbed was determined to be approximately 2.25×10-4 mg/cm2 at the plateau. Comparison of the radius of gyration of polystyrene in solvent is made to the results obtained.

Precise Coulometric Titrations of Potassium Dichromate.

Abstract: A method has been developed for the precise assay of potassium dichromate by constant-current coulometric titration with ferrous ions generated at a platinum cathode. It, is shown that one-half gram samples of the dichromate can be titrated with a standard deviation of 0.003 percent.

A Method for Determining the Elastic Constants of a Cubic Crystal from Velocity Measurements in a Single Arbitrary Direction; Application to SrTiO3.

Abstract: Three independent velocities of sound can be measured along any direction of propagation in a cubic crystal except the [100] and [111] directions. These three velocities suffice to determine the three elastic constants and for the [110] direction, the calculation of these constants is easy. For all other directions, the calculation is more difficult; the only existing method appears to be a perturbation technique developed by Neighbours. The present paper presents a method using exact equations and an iterative procedure to solve these equations and to calculate both the elastic constants and their standard deviations from the sound velocities and their standard deviations. The method is illustrated with new data on SrTiO3 which give c11=3.156±0.027, c12=1.027±0.027, c44= 1.215±0.006×1012 dynes/cm2 at 25 °C. The importance of including covariance terms in calculations of the standard deviations is emphasized.

Heat Exchange in Adiabatic Calorimeters.

Abstract: Heat flow in adiabatic calorimeters of various shapes and materials is described in terms of linear partial differential equations. From these equations it is deduced that in the intermittent heating method the heat exchange between the calorimeter and the adiabatic shield due to transients at the beginning and end of the heating period can be made to cancel. The remaining heat exchange is the same for intermittent or continuous heating methods and can be treated as the sum of effects due to gradients set up by heat flow (1) from the shield to the environment and (2) from the shield and calorimeter heaters to raise the temperatures of the shield and calorimeter, respectively. The first effect can be accounted for by measurements during fore and after periods in intermittent calorimetry and by varying the heating rate in continuous calorimetry. Under certain conditions the second effect can be accounted for by measurements with the empty calorimeter. Variation in heating rate fails as a test for the magnitude of the second effect.

An Investigation of the Constitution of the Mercury-Tin System.

Abstract: An investigation of the constitution of the mercury-tin system was made by a combination of three techniques: differential thermal analysis; diffusion and chemical analysis; and X-ray diffraction. The mercury-silver-tin system is of interest because it is the basis of dental amalgam, the most important single dental restorative material. Information as to the constitution of these alloys is incomplete for both the ternary system and the mercury-tin binary system. This study was devoted to the investigation of the mercury-tin system as a prerequisite to a study of the ternary alloys. The results obtained by the various methods are not in complete agreement. They indicate that the system is more complex than previously reported. Additional evidence for the beta phase as reported by Prytherich was found but the composition limits and eutectoid temperature remain to be confirmed. The gamma phase composition limits were found to differ from earlier values. Corroborative data for Gayler's delta phase and possible evidence for a previously unreported epsilon phase have been found by X-ray diffraction. The thermal analysis results indicate the possible existence of additional phases unconfirmed by other methods. A modified mercury-tin phase diagram based upon these findings is proposed.

Absolute Configuration and Chemical Topology.

Abstract: The stereochemistry of catenanes, knotted molecules, and Borromean rings is discussed. An augmentation of the Cahn-Ingold-Prelog convention for designating absolute configuration is proposed. A convention is proposed for designating the absolute configuration of knotted molecules. A suggestion is made concerning the citing of the absolute configuration of molecularly dissymmetric diastereomers.

Quantitative Metallography With a Digital Computer: Application to a Nb-Sn Superconducting Wire.

Abstract: Accurate quantitative data pertinent to the structure of solid materials at the micro size level, which are difficult or prohibitive to obtain by traditional manual measurements, are now obtained directly by a digital computer which uses a photomicrograph as the information input. The history of picture interpretation experiments at the National Bureau of Standards is reviewed. The fundamental computer operations are illustrated, together with a description of 24-image processing routines now functional at a practical level. A micrograph of a specimen of Nb-Sn superconductor wire is exhaustively analyzed. This specimen is found to contain approximately 70 percent Nb/sub 3/Sn, nearly all of which is mutually interconnected. It is also found that in this specimen the mean free path in the Nb/sub 3/Sn superconducting phase is only 26.5 microns. This small value results from the spongy structure of the material and numerous interruptions caused by voids and by particles of four other solid phases. The comparative importance of the several types of interruptions is measured. It is determined that small voids are the most important single cause of the short mean free path, and deduced that these voids appear to have formed mainly from the reaction during heat treatment.

High-Temperature Thermodynamic Functions for Zirconium and Unsaturated Zirconium Hydrides.

Abstract: Giving greatest weight to the experimentally measured highest decomposition pressures and the enthalpies in one-phase fields, thermodynamicall y interconsistent integral and differential enthalpies (heat contents), heat capacities, entropies, and Gibbs free energies are derived for the crystalline one- and two-phase fields of the zirconium-hydrogen system for all stoichiometric compositions from Zr to ZrHi25 and over the temperature range 29815 to 1,200 °K These properties are derived in analytical form, and in most cases are represented by numerical equations, with tabulation for zirconium and H/Zr atom ratios of 025, 050, 057, 075, 100, and 125 Most of the unique phase-field boundaries which are consistent with the derived properties are located and are compared with those previously reported In the Zr-H system the enthalpies are shown to relate certain properties at different com­ positions as well as at different temperatures Some of the various data show good interconsistency, while others reveal discrepancies which are discussed critically

Self-Broadening of Carbon Monoxide In the 2 v and 3 v Bands *

Abstract: The self-broadening of carbon monoxide has been measured for the 2 v and 3 v bands with pressures up to 3.5 atmospheres. A grating spectrometer of high resolving power was used for the measurements and the correction for finite slits was small. The corrections varied from 3 to 20 percent for the different conditions of measurement. The half-widths per atmosphere, γ°, decreased from 0.089 cm-1 for |m| = 1 to 0.053 cm-1 for |m| = 21. The half-widths are compared with those obtained by other investigators and it is shown that the results reported in this work fall in between the self-broadening values previously obtained.

Relaxation Modes for Trapped Crystal Point Defects.

Abstract: Haven and van Santen [l]1 and Wachtman [2] have used group theory methods to determine the relaxa­ tion modes appropriate to a vacancy trapped in the vicinity of an impurity ion in an ionic crystal, Wachtman applied the results to a study of Th0 2 containing Ca ++ and oxygen ion vacancies. Haven and van Santen treated the case of a sodium-chloride lattice containing divalent impurities and sodium-ion vacancies. They gave complete solutions for all modes for the relaxation involving only nearestneighbor positions, and also considered several spe­ cial cases when next-nearest-neighbor positions were allowed as well. Previously, Lidiard [3] had solved, without restrictions, for the electrically active relax­ ation involving next-nearest-neighbor positions, but had not distinguished the two distinct modes present in his solutions. Dreyfus [4] showed that Lidiard's solution was indeed equivalent to two electrically active modes. Haven and van Satnen had also ob­ tained two electrically active relaxation modes. The group theory methods can in fact be very readily extended to include any number of additional sets of neighbors. The problem of the undriven relax­ ation can be expressed mathematically in matrix form [2]: where / is the identity matrix; C is a matrix in which the element in the ith row and jth column is the negative probability per unit time of a jump from the jth into the ith site, and in which the ith diagonal element.is the sum of all jump probabilities out of the ith. site; and p is the vector whose ith component is the probability of occupation of the 'ith site by the point-defect. The eigenvectors of C in the space of -» p are then the relaxation modes.

Precise Coulometric Titrations of Halides.

Abstract: A method has been developed for the precise assay of halides by constant-current coulometric titration with silver ions generated at a silver anode. It is shown that a 4 milliequivalents sample of halide can be titrated with standard deviation of 0.005 percent.

Symmetry Splitting of Equivalent Sites in Oxide Crystals and Related Mechanical Effects.

Abstract: Changes in the symmetry of a crystal caused by an applied strain have been used to show in what circumstances an internal friction peak can result from the motion of isolated point defects. General rules are given to make the prediction, and these are applied to several structures of common oxides. The prediction for rutile is compared with experi­ mental results which are interpreted by the movement of titanium ions between interstitial positions in the structure.

Third Spectrum of Palladium (Pd III).

Abstract: The Pd III spectrum has been observed from 688 to 2991 A, and the earlier analysis has been revised and extended. The number of Pd III lines here reported is 1,110, of which 917 are classified as combinations of 57 even energy levels with 111 of odd parity. The interpretation has been aided by theoretical predictions of the approximate positions of expected energy levels. Spectral terms from the 4d 8, 4d 7 ns 1, and 4d 6 5s 1 5p 1 configurations are designated. Eight limit terms are in the 4d 7 ns 1 configurations. The earlier ionization potential, 33.0 electron volts derived from the (ns 1) 3,5 F series (n=5,6) by means of a Ritz formula, remains unchanged.

Reduction of Space Groups to Subgroups by Homogeneous Strain.

Abstract: It is assumed that the symmetry elements possessed by a strained crystal will be those common to the unstrained crystal and to the macroscopic state of strain. This principle has been applied to show all of the possible subgroups to which a given space group can be lowered by homogeneous strain for all of the 230 crystallographic space groups.

Analysis of Two Infrared Bands of CH2D2.

Abstract: Two infrared absorption bands of CH/sub 2/D/sub 2/ were analyzed in the semirigid rotor approximation. These are the Atype band at 2671.87 cm/sup -1/ and the C-type band at 4425.81 cm/sup -1/. The A-type band has previously been assigned as nu /sub 3/ + nu /sub 9/, and the C-type band is tentatively assigned as nu /sub nu /sub 6/. The upper state of the A-type band is perturbed presumably by the close lying level 2 nu /sub 5/. The following values were found for the rotational constants of the ground vibrational state: A/sub 0/ = 4.303 cm/sup -1/, B/sub 0/ = 3.504 cm/sup -1/, C/sub 0/ cm/sup -1/. (auth)

Photolytic Behavior of Silver Iodide.

Abstract: Silver iodide exposed to high intensity radiation in the visible light spectrum was found to yield a powder X-ray diffraction pattern showing marked deviations from ideality. It was found possible to correlate these with a decrease in primary extinction, indicating a constant progress from an ideal to a mosaic type crystallinity. Large single crystals showed pronounced asterism in transmission Laue photographs under similar experimental conditions. Small amounts of colloidal silver were detected. A mechanism for this process in silver iodide is proposed, in general agreement with the theory of the photographic process. The primary difference from the other silver halides appears to be a considerably slower rate, permitting the observation of a two step process in detail.

A Study of the Gas-Stabilized Arc as an Emission Source for the Measurement of Oscillator Strengths. Determination of Some Relative gf-Values for Fe I.

Abstract: A study has been made of the application of the gas-stabilized arc source to the determination of oscillator strengths in atomic spectra. In this source, a solution of salts of one or more elements may be injected and the elements excited directly in an arc plasma with steady emission of spectral intensities. The source can be taken to be characterized by local thermal equilibrium, but with a large radial temperature gradient. An experiment indicates that when two elements are introduced into the arc in a solution, the relative concentrations of the elements in the discharge are the same as in the solution within the probable experimental error. This experiment indicates one way that the arc may be used for the determination of absolute oscillator strengths. New measurements are reported of the gf-values of 105 lines of Fe I between 2900 and 4150 A, and the new values are compared with the results of earlier measurements.

Spectrophotometric determination of the thermodynamic pK value of picric acid in water at 25 °C

Abstract: The thermodynamic pK value of picric acid was determined spectrophotometrically in water containing hydrochloric acid to repress the ionization. The pK value 0.33 (K ≈ 0.46) was obtained from data at 450 mμ. Attempts to determine the pK value by potentiometric titrations of picric acid and by spectrophotometric measurements of picric acid solutions in the near-saturation range did not yield satisfactory results. The new pK value is compared with previously published values.