Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry最新文献

A high molecular weight linear polyethylene sample has been studied by adiabatic calorimetry from 10 to 380 K. Two broad temperature regions of unusual spontaneous temperature drift have been observed. The phenomena occurring around 240 K are similar to that observed in other polyethylene samples studied in this series, and are presumed to be caused by the relaxational processes in the amorphous phase. The weak exothermic behavior occurring around 160 K is presumed to be caused by the stabilization of the quenched sample.

Relative intensities of 549 U I lines observed in a dc copper arc are used to derive transition probabilities and oscillator strengths. Upper limits to lifetimes for 65 levels in neutral uranium atoms are determined.

Using the Monte Carlo method in statistical mechanics, we have simulated high density metastable states. We find that nucleation from a three dimensional fluid array to a crystalline solid is possible, but that periodic boundary conditions and the small size of the system inhibit the formation of perfect crystals. Evidence for the existence of an amorphous solid state has also been observed, and the pair correlation function of this state exhibits some of the features associated with random close-packed arrays of hard spheres. The possible relation between these simulations and the formation of glassy states in real systems is briefly discussed.

The droplet technique was used to obtain estimates of the isothermal rate of homogeneous crystal nucleation in highly supercooled melts of 8 characterized fractions of linear polyethylene (weight average molecular weights from 3,100 to 249,000). The data obtained from these experiments were analyzed in accord with current theories of homogeneous nucleation of chain folded crystals. Values for the quantity σ²σ _e , where σ and σ _e are the lateral and end-surface free energies of the crystal, were estimated as a function of molecular weight. Sample 3.2 K was found to be anomalous in its nucleation behavior. When we assume that this sample crystallizes in the extended chain form and calculate σ³ instead of σ ² σ _e the value for σ is found to be 10.57 ergs/cm² which is in reasonable agreement with the value 9.6 ergs/cm² found by other investigators for linear hydrocarbons. However, there remains the question as to whether sample 3.2 K ever underwent homogeneous nucleation. For samples 9.70, 11.74, and 23.0 K, σ²σ _e was found to increase rapidly due to a decrease in the number of cilia per chain fold as the molecular weight increases. For higher molecular weights the value for σ²σ _e levels off and the average value of σ²σ _e for samples 23.0 to 249 K was found to be 19,000 ergs³/cm⁶. The experimental value of the absolute nucleation frequency I ₀, was found to differ from the theoretical value by approximately 1 × 10¹². If one assumes that the surface free energies are temperature dependent [i.e., $\sigma ={\sigma }_1(1+\overline{x}\text{Δ}T)$ and ${\sigma }_e={\sigma }_{e1}(1+\overline{y}\text{Δ}T)$ where $\overline{x}=-0.0073$ and $\overline{y}=0.014$ ] the average value of σ²σ _e > changes only slightly (to 19,800 ergs³/cm⁶) due to the compensating effects in the signs of the temperature corrections and I ₀ is close to the theoretical value, 1 × 10³⁴ nuclei/cm³/s.

Solid sample, thermal ionization, mass spectrometry has been used to obtain absolute values for the isotopic abundance ratios of a reference sample of potassium. Standards of known isotopic composition, prepared by gravimetrically mixing nearly isotopically and chemically pure separated isotopes of, ³⁹K and ⁴¹K, were used for calibration. The absolute isotopic abundance ratios are : ³⁹K/⁴¹K = 13.8566 ± 0.0063 and ⁴⁰K/⁴¹K = 0.0017343 ± 0.0000061 which yield atom percent compositions of ³⁹K=93.2581 ± 0.0029. ⁴⁰K = 0.01167 ± 0.00004, and ⁴¹K = 6.7302 ± 0.0029. The calculated atomic weight for potassium is 39.098304 ± 0.000058. The indicated uncertainties are overall limits of error which are the sum of the uncertainty components for ratio determinations and the components covering the effects of known sources of possible systematic error.

Absolute values have been obtained for the isotopic abundance ratios of a reference sample of silicon using electron impact mass spectrometry. Samples of known isotopic composition prepared from nearly isotopically pure separated silicon isotopes were used to calibrate the mass spectrometers. The resulting absolute ²⁸Si/³⁰Si ratio=29.74320±0.00747 and the ²⁹Si/³⁰Si ratio=1.50598±0.00086 which yield atom percents of ²⁸Si = 92.22933± 0.00155, ²⁹Si = 4.66982 ± 0.00124 and ³⁰Si = 3.10085 ± 0.00074. The atomic weight calculated from this isotopic composition is 28.085526 0.000056. The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the means and allowances for the effects of known sources of possible systematic error. A study of natural ²⁸Si/³⁰Si ratio variations reported in the literature extends the estimated uncertainty in the atomic weight of natural silicon to ±0.00039.

The use of a specially designed coulometer and a high accuracy coulometric circuit resulted in the accurate measurement of the electrochemical equivalent and atomic weight of zinc. The experimental conditions to make possible the final precise and accurate measurements were established. These include a study of mechanical losses from the anode during the electrolysis and the corrosion of zinc in various media used in the determination. The effects of both of these sources of error may be controlled. Mechanical losses are minimized when an amalgamated electrode is used; corrosion, when an amalgamated electrode is used in an air free system. An electrolyte, 25 wt. percent NH₄Cl and 3 molal ZnCl₂. was used in these determinations. This report presents the account of research which was prerequisite to subsequent accurate determination of the electrochemical equivalent and the atomic weight of zinc.

Synchrotron radiation has been used as a standard source to calibrate spectrographic instruments at the National Bureau of Standards (NBS). Conceptually it is straightforward to apply the calculable continuum distribution of synchrotron radiation to problems requiring a source of known irradiance if the electron energy, the radius of the electron orbit, and the beam current are known. In practice many factors affect the accuracy of such a calibration, such as temporal and spatial variations in the electron beam, uncertainties in the orbital radius and maximum energy of the orbiting electron beam. These sources of error are discussed and the method of calibration on SURF-I is specified. A storage ring synchrotron radiation facility (SURF-II) is now operational at NBS. The calibration techniques developed for SURF-I are applied to SURF-II with anticipated improvements in calibration accuracy. For SURF-I the incident flux was determined with an accuracy of 15 percent while for SURF-II we anticipate accuracies of about 7 percent.

This note points out errors in the values of the third virial coefficients for pure water vapor which appeared in a 1967 paper by Hyland and Mason. The errors arose while converting from the units of Goff and of Keyes to the desired units of (liter/mole)². The consequences of the errors are outlined, and it is shown that there is no effect on the primary results of the paper, namely, in the preferred values of the third interaction virial coefficient for air-water vapor mixtures, C _aww .

A new, successful approach to the determination of atomic weights of suitable elements has been demonstrated in this research. An absolute constant-current coulometric method was employed for the determination of the electrochemical equivalent and the atomic weight of zinc. The effects of possible sources of systematic error were investigated and appropriate corrections applied. The newly determined values of the two constants are 0.3387958 mg C^-1 and 65.3771 respectively. The uncertainty in the atomic weight of zinc was reduced by more than an order of magnitude. The publication of partial data resulted in the revision of the value of the atomic weight of zinc by the International Union of Pure and Applied Chemistry.