For many users of the neutron and X-ray reflectometry instruments at NIST, these measurements represent a relatively small and specialized part of their research portfolio. As such, providing calculation and modeling tools that are as accessible and easy-to-use as possible is a high priority of the facility. In order to meet this need, a purely web-browser-based calculator for reflectivity modeling and rudimentary fitting has been developed and provided on a publicly accessible web server.Going to https://www.ncnr.nist.gov/instruments/magik/calculators/reflectivity-calculator.html, will load a one-page web application into the browser. Any relatively modern browser with support for ECMAScript 5 will be able to load and run the application. A calculator for magnetic samples can be found at https://www.ncnr.nist.gov/instruments/magik/calculators/magnetic-reflectivity-calculator.html.
Comparative modeling of protein structure is a process which determines the three-dimensional structure of protein molecules on the basis of amino acid sequence similarity to experimentally linown structures. The procedure is facilitated by the growing database of protein structures obtained from crystallography. In this review a series of stages in the modeling process are identified and discussed. These are: (i) obtaining a reliable amino acid sequence of the structure of interest, (ii) producing a structurally correct sequence alignment, (iii) identifying which structural features are conserved between target and parent structures, (iv) modeling the new pieces of structure, and (v) tests of reliability.
A supercritical fluid chromatograph has been designed and constructed to make physicochemical measurements, while retaining the capability to perform chemical analysis. The physicochemical measurements include diffusion coefficients, capacity ratios, partition coefficients, partial molar volumes, virial coefficients, solubilities, and molecular weight distributions of polymers. In this paper, the apparatus will be described in detail, with particular attention given to its unique features and capabilities. The instrument has recently been applied to the measurement of diffusion coefficients of toluene in supercritical carbon dioxide at a temperature of 313 K, and pressures from 133 to 304 bar (13.3-30.4 MPa). The data are discussed and compared with previous measurements on similar systems.
The resistance of metals is a function of applied pressure, and this dependence is large enough to be significant in the calibration of transient hot-wire thermal conductivity instruments. We recommend that for the highest possible accuracy, the instrument's hot wires should be calibrated in situ. If this is not possible, we recommend that a value of γ, the relative resistance change with pressure, of -2×10-5 MPa-1 be used to account for the pressure dependence of the platinum wire's resistance.
The procedures used to model a protein structure are well established when the novel protein has high sequence similarity to a protein of known structure. Many proteins of interest have low (i.e. <50%) sequence similarity to any known structure. In these cases new approaches to prediction of structure are required. The use of sequence profiles which relate sequence to known structure has been proposed as one method to assign local regions of structure. As a first stage, templates or "icons" of the many relevant substructural motifs found in proteins must be defined. The sequences which gave rise to these structures are then aligned and a weighted profile obtained. Average structures of the 8 and 12 residue helix-turn and turn-helix motifs have been prepared. These coordinate templates were then used to scan through the Brookhaven protein structural database for similar, superimposable fragments. A composite template of 100 similar fragments for each element was found to be internally consistent to a rmsd=0.92 Å for HT8, 1.54 Å for HT12, 0.41 Å for TH8 and 1.40 Å for TH12. All of the sequences, from these structures, were then used to create an overall sequence profile. The four sequence profiles were scanned against the amino acid sequences of the proteins in the Brookhaven database: tertiary structure was correctly identified only about 10% of the time. This value is too low for predictive purposes. However, it could be increased by checking for multiple occurrences of the template in one protein.
Scattering parameter expressions are developed for the principal mode of a coaxial air line. The model allows for skin-effect loss and dimensional variations in the inner and outer conductors. Small deviations from conductor circular cross sections are conformally mapped by the Bergman kernel technique. Numerical results are illustrated for a 7 mm air line. An error analysis reveals that the accuracy of the scattering parameters is limited primarily by the conductor radii measurement precision.
Numerical database systems have recently become available online.Their enhanced search capabilities and fast retrieval of data make them a valuable tool in research.In particular, CRYST-DAT which is a search and analysis system for NBS CRYSTAL DATA has proven to be powerful in the identification of crystalline materials.In con-junction with a single-crystal x-ray diffractometer, a qualitative as well as quantitative phase determination is easily performed.The use of CRYSTDAT will be illustrated in several examples.
Databases for use with analytical chemistry instrumental techniques are surveyed, with attention to existing databases and collection efforts now underway, as well as needs for new data-bases. Collections of spectra for use in NMR, infrared spectroscopy, and mass spectroscopy are described. Using mass spectral databases as an example, a critique is presented of automated quality control procedures used to evaluate individual spectra in large collections; the kinds of problems which have been en-countered in using these procedures are discussed. Finally, a brief critical review is presented covering the application of computers to the identification of unknown compounds using spectral data-bases; again, algorithms used with mass spectrometry are taken as the example. Ongoing work at NIST with the NIST/EPA/MSDC Mass Spectral Database is concerned with many of these problems; recent developments are described.
The development of a computerized database on advanced structural ceramics can play a critical role in fostering the widespread use of ceramics in industry and in advanced technologies. A computerized database may be the most effective means of accelerating technology development by enabling new materials to be incorporated into designs far more rapidly than would have been possible with traditional information transfer processes. Faster, more efficient access to critical data is the basis for creating this technological advantage. Further, a computerized database provides the means for a more consistent treatment of data, greater quality control and product reliability, and improved continuity of research and development programs. A preliminary system has been completed as phase one of an ongoing program to establish the Structural Ceramics Database system. The system is designed to be used on personal computers. Developed in a modular design, the preliminary system is focused on the thermal properties of monolithic ceramics. The initial modules consist of materials specification, thermal expansion, thermal conductivity, thermal diffusivity, specific heat, thermal shock resistance, and a bibliography of data references. Query and output programs also have been developed for use with these modules. The latter program elements, along with the database modules, will be subjected to several stages of testing and refinement in the second phase of this effort. The goal of the refinement process will be the establishment of this system as a user-friendly prototype. Three primary considerations provide the guidelines to the system's development: (1) The user's needs; (2) The nature of materials properties; and (3) The requirements of the programming language. The present report discusses the manner and rationale by which each of these considerations leads to specific features in the design of the system.
This report provides the background for and summarizes the main results of the 18th meeting of the Consultative Committee on Electricity (CCE) of the International Committee of Weights and Measures (CIPM) held in September 1988. Also included are the most important implications of these results. The principal recommendations originating from the meeting, which were subsequently adopted by the CIPM, establish new international reference standards of voltage and resistance based on the Josephson effect and the quantum Hall effect, respectively. The new standards, which are to come into effect starting January 1, 1990, will result in improved uniformity of electrical measurements worldwide and their consistency with the International System of Units or SI. To implement the CIPM recommendations in the U.S. requires that, on January 1, 1990, the value of the U.S. representation of the volt be increased by about 9.26 parts per million (ppm) and the value of the U.S. representation of the ohm be increased by about 1.69 ppm. The resulting increases in the U.S. representations of the ampere and watt will be about 7.57 ppm and 16.84 ppm, respectively. The CCE also recommended a particular method, affirmed by the CIPM, of reporting calibration results obtained with the new reference standards that is to be used by all national standards laboratories.
Two decades have passed since the first large scale, public access computer-based information systems were developed to store and disseminate the knowledge of medicine and biology. These first systems were bibliographic, and though the searching of computer files of citations remains the most common use of biological databases, there are dramatic forces at work in basic biology which are driving a transition from the printed page to the factual database. Unlike bibliographic systems, which contain only a pointer to information located elsewhere, factual data-bases contain the information sought. Development of automated methods to sequence DNA, RNA, proteins, and other macromolecules have yielded oceans of cryptic symbols, for which there is an absolute dependence upon computerized factual databases to acquire, store, retrieve, and analyze data. The Human Genome Project has focussed attention on the information science aspects of nucleic acid data, yet for the practicing scientist nucleic acids and other sequence data are just one piece of an increasingly complex biological puzzle whose solution will be expressed in terms of structure and function. Access to and integration of information across multiple related biological data-bases is a major challenge facing information system builders, a challenge which holds the promise of creating knowledge synergy from what are today disconnected, stand-alone information sources.
A brief overview of recent superconductivity research at NIST is presented. Emphasis is placed on the new high-temperature oxide superconductors, though mention is made of important work on low-temperature superconductors, and a few historical notes are included. NIST research covers a wide range of interests. For the new high-temperature superconductors, research activities include determination of physical properties such as elastic constants and electronic structure, development of new techniques such as magnetic-field modulated microwave-absorption and determination of phase diagrams and crystal structure. For the low-temperature superconductors, research spans studying the effect of stress on current density to the fabrication of a new Josephson junction voltage standard.
Computational molecular biology is a relatively new specialty that has arisen in response to the very large amount and quality of data currently being produced, including gene and protein sequences ("one-dimensional" information) and nucleic acid and protein structures ("three-dimensional" information). Many important biological investigations can be carried out only through effective computational access to the entire corpus of data. This has stimulated the development of data banks and information retrieval systems. For example, after determination of a new gene sequence, one would like to know whether it is possible to say anything about its structure and function. To try to answer this question one screens the sequence of the corresponding protein for a significant similarity to a protein of known structure. In this article we shall describe the kinds of inferences that are possible if such a relationship is found.
A new database containing crystallographic and chemical information designed especially for application to electron diffraction search/match and related problems has been developed. The new database was derived from two well-established x-ray diffraction databases, the JCPDS Powder Diffraction File and NBS CRYSTAL DATA, and incorporates 2 years of experience with an earlier version. It contains 71,142 entries, with space group and unit cell data for 59,612 of those. Unit cell and space group information were used, where available, to calculate patterns consisting of all allowed reflections with d-spacings greater than 0.8 A for ~ 59,000 of the entries. Calculated patterns are used in the database in preference to experimental x-ray data when both are available, since experimental x-ray data sometimes omits high d-spacing data which falls at low diffraction angles. Intensity data are not given when calculated spacings are used. A search scheme using chemistry and r-spacing (reciprocal d-spacing) has been developed. Other potentially searchable data in this new database include space group, Pearson symbol, unit cell edge lengths, reduced cell edge length, and reduced cell volume. Compound and/or mineral names, formulas, and journal references are included in the output, as well as pointers to corresponding entries in NBS CRYSTAL DATA and the Powder Diffraction File where more complete information may be obtained. Atom positions are not given. Rudimentary search software has been written to implement a chemistry and r-spacing bit map search. With typical data, a full search through ~ 71,000 compounds takes 10~20 seconds on a PDP 11/23-RL02 system.
Scientific numeric databases are important research tools for materials scientists. In distinction to bibliographic databases, these numeric databases are useful primarily to provide direct, immediate access to data, often evaluated data. Examples showing the application of crystallographic databases are given, including determining candidate materials for certain applications. Thermo-chemical data useful for optimizing optical fiber processing are discussed showing the importance of high-quality data. In addition, these databases are an important tool that can be utilized in the graduate education of the next generation of materials scientists.
The National Institute of Standards and Technology (NIST) calibration service for voltage transformers and high-voltage capacitors is described. The service for voltage transformers provides measurements of ratio correction factors and phase angles at primary voltages up to 170 kV and secondary voltages as low as 10 V at 60 Hz. Calibrations at frequencies from 50-400 Hz are available over a more limited voltage range. The service for high-voltage capacitors provides measurements of capacitance and dissipation factor at applied voltages ranging from 100 V to 170 kV at 60 Hz depending on the nominal capacitance. Calibrations over a reduced voltage range at other frequencies are also available. As in the case with voltage transformers, these voltage constraints are determined by the facilities at NIST.
SURFACE METROLOGY A Report on the Fourth International Conference on the Metrology and Properties of Engineering Surfaces, National Bureau of Standards, Gaithersburg, MD, April 13-15, 1988 carbon black agglomerates in rubber, a family of materials that might normally be thought of as too soft for reliable characterization by stylus. Inadequate dispersion leads to reduced product life and other detrimental effects in rubber products. Figure 1 shows the stylus in contact with such a rubber surface. The presence of the agglomerates is evidenced either by discrete peaks in the stylus profiles or by valleys left when the material is separated during the cutting process.
In this paper, we will discuss the interface between expert systems and laboratory robotics.We will use examples from our recent research to illustrate how we are building an effective interface and indicate where we think this research will lead.What are expert systems?As an operational definition we will adopt the following:Expert Systems are a sub-specialty in artificial intelligence (AI).The term is generally understood to mean a "knowledge-based" or "knowledge-driven" system designed to represent and apply factual knowledge in specific, very limited areas of expertise.
