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

The enthalpies of combustion and formation of two samples of linear polyethylene which differ only in the degree of crystallinity have been determined in an oxygen bomb calorimeter. For the two samples the degree of crystallinity, the enthalpy of combustion at 298.15 K, and the enthalpy of formation at 298.15 K, were respectively: 72 percent, -651.16 ± 0.12 kJ · mol^-1, -28.18 ± 0.13 kJ · mol^-1 for the less crystalline sample; and 96 percent, -650.27 ± 0.12 kJ · mol^-1 and -29.08 ± 0.12 kJ · mol^-1 for the more crystalline sample. The values are per mole of CH₂. Uncertainties listed are estimates of accuracy of approximate 95 percent confidence limits. The results of previous determinations by other investigators are discussed briefly.

Wavelengths, relative intensity estimates, and classifications of the spectral lines of doubly ionized praseodymium (Pr iii) in the range of 2107 to 10716 Å are given. About 4400 lines are included of which some 2500 are classified.

A new type of 3-body calorimeter for measuring absorbed dose produced by ionizing radiation is described in detail. All three bodies rise in temperature during irradiation, and the heat absorbed by the central core is measured by standard means. Only the central core is heated during electrical calibration, but the increased heat losses are compensated by measuring most of the heat lost to the surrounding jacket and automatically adding it to the heat retained by the core. The third body is a massive, thermally-floating shield, whose presence reduces the heat losses during irradiation, with a consequent increase in sensitivity and stability. A mathematical description of the calorimeter behavior is presented, along with a discussion of control and operation technique. In particular, it is shown how this 3-body calorimeter can be calibrated as a 1-body calorimeter, with large heat losses, or as a 2-body calorimeter, in the quasi-adiabatic mode. This calorimeter design decreases the effects of thermal gradients and at the same time provides the means to test for these effects. The results of these tests show that for this particular model, systematic errors caused by thermal gradients, during electrical measurements, are no larger than 0.1 percent. Errors in comparing an electrical run with an irradiation may be somewhat larger because of different temperature gradients within the system. It is also pointed out that the general design of this calorimeter is not restricted to measuring absorbed dose but can be applied to calorimetry in general.

Dilatometric and ultrasonic measurements were made on mixtures of pentane and 2-methylbutane to give density, relative volume, isothermal bulk modulus, velocity of sound, and adiabatic bulk modulus to pressure of 24 kilobars (2.4 ×10⁹ N/m²).

Long-time creep of natural rubber cured with a sulfur-accelerator recipe containing no filler can be conveniently represented by a plot of (E - E ₁)/E ₁ = ΔE/E ₁ with a double-abscissa scale showing log t and t. E is the elongation at any time t, after application of the load, and E ₁ its value at unit time. Experimental data conform to the equation except for a more rapid rise preceding rupture. The constants A and B can be evaluated from only three observations-at the longest time (about 70 days), at one minute, and at an intermediate time. ΔE/E ₁ is approximately linear with log t when t is less than 0.1(A/B) and approximately linear with t when t is greater than 4.343(A/B). The observed modulus was about 1.4 MPa and A was about 2.4 percent/(unit log t) when the atmosphere was a vacuum, dry N₂, or dry air. The modulus was lowered very slightly and A became about 4 percent/(unit log t) when the air was saturated with water. B was raised from about 2 × 10^-5 percent/min to about 20 × 10^-5 percent/min when the vacuum or dry N₂ was replaced by dry air and to about 50 × 10^-5 percent/min when the air was saturated with moisture. A is considered to be related to physical relaxation, while B corresponds to a chemical reaction, probably oxidative degradation.

The enthalpy of KCl relative to that at 273.15 K was precisely measured by drop calorimetry from 273 to 1174 K, and smooth thermodynamic functions were derived for this temperature range. The heat capacities found for the crystalline phase join smoothly the most precise published data for lower temperatures; those for the liquid phase are temperature-independent within the precision of measurement over the 120° range covered. It is concluded that the broad exponential upturn of the heat-capacity curve below the melting point, if attributed to lattice vacancies, indicates a predominance of large vacancy clusters.

Published data are used to develop factors for predicting the viscosity-temperature relationship from the compositions of "soda-lime" type silicate glasses at specific temperatures in the range of 600 to 1300 °C. The effects of Na₂O, K₂O, CaO, MgO, Al₂O₃ and their interactions are evaluated. The influence of minor amounts of BaO, B₂O₃, Li₂O, and F₂, in the temperature range of 700 to 1300 °C, is estimated.

An apparatus is described which measures the equilibrium distribution of a hydrocarbon between a gas phase and a liquid water phase. The method involves a multiple equilibration procedure which requires the analysis of only the gas phase. Gas-liquid chromatography was used for the hydrocarbon analysis because of its high sensitivity and selectivity. Supplemented by vapor pressure data, the observed distribution can be used to calculate the solubility of the hydrocarbon in the liquid phase. This was done for benzene, toluene, and ethylbenzene in distilled water over the temperature range 5 to 20 °C and in an artificial seawater over the temperature range 0 to 20 °C. The various factors affecting the accuracy of the results are discussed in detail.

The system NaCl-AlCl₃ has been restudied by DTA, visual observation, and x-ray diffraction powder techniques for identification of crystalline phases. It was confirmed that the system contains one intermediate compound NaAlCl₄ with an incongruent mp of 153 ± 0.5 °C and a region of liquid immiseibility extending from 80.25 to 99.6 mol percent AlCl₃ at 191.3 °C, the monotectic temperature.

Simultaneous measurements of heat capacity, electrical resistivity and hemispherical total emittance of zirconium in the temperature range 1500 to 2100 K by a subsecond duration, pulse heating technique are described. The results are expressed by the relations: $C_p=36.65-1.435\times {10}^{-2}T+6.624\times {10}^{-6}{T}^2$ $\rho =87.95+1.946\times {10}^{-2}T$ $ϵ=0.2031+6.362\times {10}^{-5}T$ where C _p is in J · mol^-1 · K^-1, ρ is in 10^-8 Ω · m, and T is in K. Estimated inaccuracies of the measured properties are: 3 percent for heat capacity, 2 percent for electrical resistivity and 5 percent for hemispherical total emittance.