scispace - formally typeset
Search or ask a question
JournalISSN: 0022-4332

Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry 

The National Institute of Standards and Technology
About: Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry is an academic journal. The journal publishes majorly in the area(s): Vapor pressure & Enthalpy. Over the lifetime, 882 publications have been published receiving 29341 citations.


Papers
More filters
Journal ArticleDOI
TL;DR: In this paper, an evaluated compilation of equilibrium relative humidity in air versus temperature from pure phase to approximately 105 pascal (1 atm) in pressure is presented for 28 binary saturated aqueous solutions.
Abstract: An evaluated compilation of equilibrium relative humidities in air versus temperature from pure phase to approximately 105 pascal (1 atm) in pressure is presented for 28 binary saturated aqueous solutions. The relative humidities of the solutions range from about 3 to 98 percent. Using a data base from 21 separate investigations comprising 1106 individual measurements, fits were made by the method of least squares to regular polynomial equations with two through four coefficients. Equations and tables are presented along with the estimated uncertainties in the correlated results. Research, hygrometer calibration, testing and material conditioning often require the accurate control of humidity in a working space. The common methods of controlling the humidity accurately use either a humidity generator (1A)1 or the equilibration of a closed space with a chemical system (IB) which produces the desired equilibrium vapor pressure. Humidity generators tend to be expensive and complex whereas equilibration with chemical systems that provide fixed points is a relatively inexpensive and simple method of humidity control. Among the chemical systems used for this purpose are aqueous sulphuric acid solutions, glycerine and water solutions and single and binary salt solutions. Each such solution offers a degree of humidity adjustment that can be achieved by changing its concentration. On the other hand, special problems are associated with the use of solu- tions because their concentrations must be measured and controlled. Not only must the concentration of the solution be determined initially but the presence of any humidity sources or sinks in the controlled space and even the initial equilibra- tion process of the space can alter the solution concentration. An especially useful method of humidity control by chemi- cal system involves the use of binary saturated aqueous solutions (primarily of single salts) in which the solute is highly non-volatile. At any temperature, the concentration of a saturated solu- tion is fixed and does not have to be determined. By provid- ing excess solute, the solution will remain saturated even in the presence of modest sources or sinks. Where the solute is a solid in the pure phase, it is easy to determine that there is indeed saturation. Due to the ease of its use, this is a popular method of humidity control. Since a given saturated salt solution provides only one relative humidity (RH) at any desired temperature, a different relative humidity must be achieved by selecting another appropriate salt. Though much data on saturated salt solu-

3,503 citations

Journal ArticleDOI
TL;DR: It is concluded that methods involving a variable rate of heating or involving several thermogravimetric traces at different rates of heating are capable of establishing the uniqueness of kinetic parameters.
Abstract: Theoretical equations are developed for typical decompositions of polymers including those in which the volatilization does not follow a simple "reaction order" and those made up of a composite of several reactions of differing energies of activation. The effects of order, activation energy, heating rate and temperature dependence upon the calculated thermograms is illustrated. The literature on thermogravimetric kinetics is critically reviewed and coalesced into a logical and coherent development stressing the interrelation of methods and employing a consistent system of notation. As a result, a number of improved methods and new methods for the analysis of kinetic data applicable to the complex systems mentioned above are developed. It is concluded that methods involving a variable rate of heating or involving several thermogravimetric traces at different rates of heating are capable of establishing the uniqueness of kinetic parameters. A new method of determining initial parameters from rate-conversion data is developed. A novel concept is employed of programming reaction variables (in this case, the heating rate) in a manner which greatly simplifies the mathematics of the kinetic system and which shows promise of a wide range of applicability in the area of rate processes.

1,644 citations

Journal ArticleDOI
TL;DR: In this paper, a new method of estimating the equilibrium melting temperature, Tm, of a polymer is described, and applied to polychlorotrifluoroethylene (PCTFE), where the data fit a straight line of positive slope on a Ti (obs) versus Tx plot, Tx being the abscissa.
Abstract: A new method of estimating the equilibrium melting temperature, Tm, of a polymer is described, and applied to polychlorotrifluoroethylene (PCTFE). Experimentally determined values of the so-called observed melting point, !F^(obs), are plotted as a function of the isothermal crystallization temperature, Tx. When freed of secondary effects, such as recrystallization, the data fit a straight line of positive slope on a Ti (obs) versus Tx plot, Tx being the abscissa. This line is then extrapolated to its intersection with the line T^ (obs) = Tx. The temperature at this intersection is Tm. This intersection is at 224 °C for PCTFE, and Tm is quoted as 224± 1 °C. (The highest melting point actually attained for a specimen was 218.2 °C.) The value of Tm estimated using the extrapolation procedure is compared with that estimated using the customary method of slow stepwise warming. A theoretical justification is given for making the type of plot mentioned above. The most important assumption used in the theory is that one of the dimensions of the growing crystal retains a value rather close to that of the appropriate growth nucleus during an isothermal crystallization, the other two dimensions being large in comparison. Combination of this with the fact that the relevant dimension of the growth nucleus will vary as the reciprocal of the degree of supercooling leads to the prediction of melting points that increase linearly with crystallization temperature. The assumption that one of the dimensions of the crystal retains a value fairly close to that of a growth nucleus can readily be justified on the basis of polymer crystal growth with chain folds. Its justification in the case of the customary bundlelike mode of crystallization is less clear. It is demonstrated experimentally that even the largest detectible crystals in PCTFE are only about 70 percent thicker than a primary nucleus, when secondary effects are minimized. The application of the theory to systems other than PCTFE is discussed briefly, and some preliminary measurements on polyethylene mentioned. Some points relating to the shape of the melting curves of highly crystalline polymers are also brought out.

1,196 citations

Journal ArticleDOI
TL;DR: The theory given for loop nuclei is both general and precise enough at the critical points to suggest that, on crystallization from sufficiently dilute solution, crystals of a definite step height are commonly to be expected for other crystallizable linear polymers than polyethylene, provided loop formation is sterically possible.
Abstract: A detailed interpretation of the kinetics of homogeneous nucleation and growth of crystals of a linear homopolymer from dilute solution is given. The probability of forming both nuclei with folded chains, and conventional bundlelike nuclei, from dilute solution is analyzed. It is predicted that at sufficiently high dilution, critical nuclei of length lp* will be formed from single polymer molecules by sharp folding of the chain backbone. The step height of the nucleus is given approximately by lp*=4σe/Δf . Here σe is the free energy required to form a unit area of the loop-containing end surfaces, and Δf is the free energy difference per unit volume of crystal between the crystalline and solution states. The quantity Δf is approximately proportional to the degree of supercooling ΔT. The growth of these nuclei is then analyzed. After growth, the resulting crystal is flat and platelike, the loops formed by the chain folds being on the upper and lower surfaces. Kinetic factors determine that the distance between the flat surfaces in the grown crystal will vary over only a narrow range about a value that is in the vicinity of 1*=4σe /Δf. (Neglecting effects due to edge free energies, the theoretical upper and lower limits are 1*=4σe /Δf and 1*=2σe /Δf, respectively.) In some cases the predicted temperature dependence of the step height of the grown crystal, 1* = const./ΔT, may be modified by the existence of a constant term resulting from the presence of an edge free energy ϵp . A grown loop-type crystal is predicted to be stable in comparison with a bundlelike crystal of the same shape and volume in a sufficiently dilute solution. The logarithm of the nucleation rate is approximately proportional to 1/(ΔT)2 near the melting point. The exponent n in the free growth rate law is predicted under various assumptions. To the extent that comparison is possible, the predictions given agree with the experimental results obtained by Keller and O'Connor and others on single crystals of unbranched polyethylene grown from dilute solution. A survey is given of homogeneous nucleation in bulk polymers, where the conventional bundlelike nucleus containing segments from many different molecules is valid, and the essential results compared with those calculated for the dilute solution case. The theory given for loop nuclei is both general and precise enough at the critical points to suggest that, on crystallization from sufficiently dilute solution, crystals of a definite step height are commonly to be expected for other crystallizable linear polymers than polyethylene, provided loop formation is sterically possible.

631 citations

Journal ArticleDOI
TL;DR: The Poincaré sphere representation of the state of polarization of light is developed and used to describe the reflection process and 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.
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.

557 citations

Network Information
Related Journals (5)
Journal of the Optical Society of America
12.4K papers, 393.2K citations
72% related
Transactions of The Faraday Society
8.9K papers, 268.8K citations
68% related
The Journal of Physical Chemistry
51K papers, 1.9M citations
67% related
Physics Today
10.7K papers, 525.6K citations
66% related
Journal of Chemical Physics
140.9K papers, 6.6M citations
66% related
Performance
Metrics
No. of papers from the Journal in previous years
YearPapers
197711
197643
197531
197439
197340
197255