Journal of research of the National Bureau of Standards (1977)最新文献

Stable law distributions occur in the description of the linear dielectric behavior of polymers, the motion of carriers in semi-conductors, the statistical behavior of neurons, and many other phenomena. No accurate tables of these distributions or algorithms for estimating the parameters in these relaxation models exist. In this paper we present tables of the functions $Q_{\alpha }\left(z\right)=\frac{1}{\pi }{\int }_0^{\infty }{e}^{-u^{\alpha }}\cos \left(zu\right)du{V}_{\alpha }\left(z\right)=\frac{1}{\pi }{\int }_0^{\infty }{e}^{-u^{\alpha }}\sin \left(zu\right)du$ together with related functional properties of zQ _α (z). These are useful in the estimation of the parameters in relaxation models for polymers and related materials. Values of the integral Q _α (z) are given for α = 0.01,0.02(0.02)0.1(0.1)1.0(0.2)2.0 and those of V _α (z) are given for α = 0.0(0.01)0.1(0.1)2.0. A variety of methods was used to obtain six place accuracy. The tables can be used to sequentially estimate the three parameters appearing in the Williams-Watts model of relaxation. An illustration of this method applied to data in the literature is given.

Indentation constitutes one of the most powerful test techniques for the systematic investigation of deformation and fracture responses in brittle materials. Indentations can be used to evaluate critical mechanical parameters (toughness, hardness, elastic modulus) with great simplicity and high accuracy.

A simple and economical procedure for accurate determinations of toughness and lifetime parameters of ceramics is described. Indentation flaws are introduced into strength test pieces, which are then taken to failure under specified stressing and environmental conditions. By controlling the size of the critical flaw, via the contact load, material characteristics can be represented universally on "master maps" without the need for statistical considerations. This paper surveys both the theoretical background and the experimental methodology associated with the scheme. The theory is developed for "point" flaws for dynamic and static fatigue, incorporating load explicitly into the analysis. A vital element of the fracture mechanics is the role played by residual contact stresses in driving the cracks to failure. Experimental data on a range of Vickers-indented glasses and ceramics are included to illustrate the power of the method as a means of graphic materials evaluation. It is demonstrated that basic fracture mechanics parameters can be measured directly from the slopes, intercepts and plateaus on the master maps, and that these parameters are consistent, within experimental error, with macroscopic crack growth laws.

The thermodynamics of stressed crystals that can change phase and composition is examined with particular attention to hypotheses used and approximations made, Bulk and surface conditions are obtained and for each of them practical expressions are given in terms of experimentally measurable quantities. The concept of open-system elastic constants leads to the reformulation of internal elastochemical equilibrium problems into purely elastic problems, whose solutions are then used to compute the composition distribution. The atmosphere around a dislocation in a cubic crystal is one of several examples that are completely worked out. The effects of vacancies and their equilibrium within a solid and near surfaces are critically examined, and previous formulas are found to be first order approximations. Consequences of the boundary equations that govern phase changes are studied with several examples. Finally, problems connected with diffusional kinetics and diffusional creep are discussed.

An apparatus is described which can be used for PVT and compressibility measurements on supercritical fluids from near room temperature to 600 °C and pressures to 35 MPa. Two separate experimental techniques are employed to obtain PVT data over a broad range of the state surface. Burnett expansions are performed to generate compressibility factor (or equivalently density) data along a well-behaved supercritical isotherm. A series of isochoric measurements is then made to extend the temperature range. Densities assigned to the isochores are determined from their intersection with the previously measured Burnett isotherm or gravimetrically. A computer is used for experimental control and for data logging. Isochoric measurements lasting several days can be performed routinely and without operator attention. The apparatus has been tested on propane to a temperature of 325 °C. The density data, estimated accurate to ±0.1 percent, are in excellent agreement with other existing data.

The longwall method of mining in underground coal seams is very efficient in uniform seams, but coal seam anomalies can make the method unprofitable and unsafe. This paper describes the theoretical basis for detection of coal seam anomalies using medium frequency (MF) radio transmission over paths on the order of 200 m in length. The key to the method is the sensitivity of the attenuation rate of the coal seam mode of propagation to changes in the coal seam parameters, such as height or electrical conductivity. From a large number of transmission paths, the principles of tomography can be used to reconstruct an image of the seam.

In 1980, the research program in pH was re-established at the National Bureau of Standards (NBS). This report describes the state of this research, as well as the state of the NBS pH standards. The thermodynamic definition and the determination of pH are elaborated. The problems of liquid junction potentials encountered in the practical determination of pH are discussed. The goal of the research program in pH is to develop and maintain a unified pH scale based on clearly stated thermodynamic criteria, with a wide range of applicability to practical pH measurements.

Two sets of high-temperature platinum resistance thermometers of different design have been tested in the temperature range 0 to 1100 °C. One set was constructed at the National Institute of Metrology, in the People's Republic of China, and the other at the National Bureau of Standards. The results of the tests provide information on long- and short-time thermometer stability, and on other characteristics such as temperature coefficient, immersion, self-heating effect, electrical leakage, and durability. The results also show that the behavior of the two sets is similar enough to allow them to be considered as a single set of thermometers, and that the sets perform as well as, or better than, other sets of thermometers tested earlier. It is expected that this information will aid in the evaluation of the high-temperature platinum resistance thermometer as an interpolating instrument for a practical temperature scale up to the gold point.

The thermal expansion coefficient α of liquid normal hydrogen (n-H₂) was measured between 18.8 and 22.2 K in the pressure range 5 to 70 bar. The results are compared with those derived from PVT measurements by others on both normal and para (p-H₂) hydrogen. Our analysis of the earlier normal data includes fitting an empirical equation of state, and expansion coefficients are derived from this equation by differentiation. We discuss the effects on α and the compressibility β from molecular quadrupole interactions; both theoretical and empirical results suggest these to be on the order of 2% or less for the normal spin mixture. We conclude that our thermal expansion data are consistent with earlier results on both n-H₂ and p-H₂ in this range of pressures and temperatures.