I. INTRODUCTION

The NIST WWW High Temperature Superconductors database (WebHTS) provides evaluated thermal, mechanical, and superconducting property data for oxide superconductors. The range of materials covers the major series of compounds derived from the Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Tl-Sr-Ca-Cu-O, and La-Cu-O chemical families, along with numerous other variants of the cuprate and bismuthate materials that are known to have superconducting phases. The materials are described by specification and characterization information that includes processing details and chemical compositions. Physical characteristics such as density and crystal structure are given in numeric tables. All measured values are evaluated and supported by descriptions of the measurement methods, procedures, and conditions. In all cases, the sources of the data are fully documented in a comprehensive bibliography.

II. BACKGROUND

Superconductors are materials in which the electrical resistivity is zero when the temperature of the material is less than a critical temperature, Tc. These materials commonly are divided into two broad types, known informally as "conventional superconductors" and "high-temperature superconductors". The latter superconductors also are called high-Tc materials [1].

Superconductors have been known only since the early part of the twentieth century [2]. The earliest superconductors were all of the type called conventional superconductors. The first known superconductor, discovered in 1911, was mercury which became superconducting at the temperature of liquid helium (4.2 K) [3]. Since that time, many other superconducting metals and metallic compounds have been discovered. As recently as the 1970s, Nb3Ge was found to have Tc = 22.3 K, a temperature that reigned as the highest Tc for more than a decade [4]. A conceptual understanding of all of the conventional superconductors was established in 1957 by the BCS theory of superconductivity [5]. This theory successfully accounted for the known phenomena of superconductivity until the dramatic discovery of oxide superconductors in the mid-1980s.

The era of high-Tc materials began in April 1986 when lanthanum barium copper oxide was reported to be superconducting at a temperature of 30 K [6]. The excitement created by this discovery was three-fold: the critical temperature was the highest yet found; the material was not normally metallic; and the BCS theory could not account for the superconductivity of this material.

That event stimulated an immediate and intense research effort worldwide that has resulted in tens of thousands of reports on the processing, characterization, and performance of these materials under a wide variety of compositions and conditions. Values of Tc have now been observed at temperatures as high as 150 K.

The intense interest in HTS materials is a direct result of their potential impact on science, technology, and economic world markets [7]. Scientifically, theories of conventional superconductivity do not appear to be adequate to explain the HTS phenomena. Technologically, use of HTS materials could reduce energy consumption in the United States by as much as 2.6 x 1018 J/yr compared to the use of conventional technologies; that amount of electrical energy could be used every year to power a city the size of Chicago. Economically, early commercial applications may occur in such areas as superefficient motors and high speed computers; the production of intense magnetic fields may yield significant advances in materials processing technology for electronic and structural ceramics; and nondestructive measurement devices such as magnetic resonance scanners could significantly enhance medical and scientific instruments.

An enormous amount of information about high-Tc materials is now available in publicly accessible sources, and numerous computerized bibliographic databases have been established to keep track of this information. Those systems allow researchers to identify potential sources of data by means of computerized searches of key words or phrases that occur in the titles and abstracts of the reports. However, access to the actual property data remains a substantial task because of the physical necessity of retrieving a large number of reports that subsequently must be reviewed and evaluated. Most importantly, users of the data must contend with several critical concerns pertaining to the reliability of the results, the adequacy of the materials characterization, the validity of the measurement procedures, and the need to assess conflicting reports. Such concerns are a serious impediment to the commercialization of the technologies associated with the new materials.

The NIST WebHTS database has been developed to alleviate some of the burdens and costs associated with those concerns by providing a single comprehensive reference of evaluated data for high temperature superconductors. Sources of data have been selected and reviewed to provide broad ranges of material compositions, characterization information, and property data. For the first version of the WebHTS, the emphasis has been placed on the oxide superconductors which have received, by far, the greatest amount of attention in the worldwide research effort. The database addresses the full range of information that is needed in the development of new or modified materials and for the application of the materials in devices exploiting the unique properties of these materials [8]. Thus, the database includes specification and processing information for the materials as well as numeric data on their thermal, mechanical, and superconducting properties.

III. CONVENTIONS

It is not unusual that a new, rapidly developing field in materials science and engineering generates a variety of terms to identify the new materials and to emphasize their distinct characters. The inherent lack of standardization in this situation can lead to confusion when comparing results from different studies. Such is the case in the study of high-Tc materials. Fortunately, two approaches to the informal designation of a superconducting material have enjoyed considerable popularity, although with numerous variations on their specific representations. In the WebHTS database, the generic terms, chemical family and informal name, are used for these popular material designations.

In WebHTS, the chemical family is a specification derived from various similar designations in common usage in the technical literature on high-Tc materials. The principal elements in the chemical composition are given in a list with the elements separated by hyphens. The order of the elements follows the most prevalent practice in the literature. For example, YBa₂Cu₃O_7-x belongs to the Y-Ba-Cu-O family. Many high-Tc materials are variations on a basic composition in which a fraction of the elements are replaced by other compatible elements. For example, iron replaces some of the copper in YBa₂Cu_3-yFe_yO_7-x. These substitutional elements are indicated in the chemical family designation by listing the substitutional elements in parentheses after the primary element. In the example with iron substituting for some of the copper, the chemical family would be Y-Ba-Cu(Fe)-O.

The informal name used in the HTS is also a specification derived from various similar designations in common usage in the literature. In the HTS, the informal name has the general format of "element:numbers", such as Y:123. The element is the first element of the common formula or chemical family name, and the integers refer to the relative amounts of the elements in the basic formula, other than oxygen. This designation is not unique, as may be seen from the examples of YBa₂Cu₃O_7-x and YBa₂Cu_3-yFe_yO_7-x both of which may be described as Y:123 materials. The informal name is useful as a quick, shorthand notation for indicating similar material structure and composition combinations and for designating compositional variations within a chemical family, such as the Y:123, Y:124, and Y:247 variations in the Y-Ba-Cu-O family.

The WebHTS database also uses another popular but imprecisely defined term, structure type, for a special purpose. In the HTS, the structure type specification is used to describe the bulk material as being a single crystal, a polycrystalline material, a noncrystalline (glassy) material, or a mixed material consisting of substantial crystalline and noncrystalline regions. This specification is important, for example, for understanding the anisotropy of certain material properties.

IV. THE USER'S INTERFACE TO THE WebHTS DATABASE

The function of the WebHTS interface is to provide access to the materials property data. The process of accessing the data is divided into three operations: specifying search criteria, conducting the search, and examining the records found by a search.

While there are many descriptors that can be defined for materials specification, only a few of those descriptors are currently of practical utility in searching the records from a broad spectrum of diversely conducted research programs. The WebHTS interface currently uses only the three most significant descriptors for search criteria. These criteria, chemical family, informal name, and structure type, are contained in the three drop-down selection boxes. These three boxes are joined together by logical AND operations. As a result, records retrieved by a search will satisfy all of these specifications simultaneously.

To specify a chemical family, an informal name, or a structure type, activate the pertinent drop-down list box by clicking the arrow button on the right of the selection box. A list of allowed choices appears below the selection box when the arrow button is clicked. Find the desired entry, and click on that entry with the mouse. The selected entry appears in the selection box.

The Properties drop-down list box may be used to indicate the properties for which information is requested. When a search is conducted, any record containing the requested properties will be identified as a candidate for viewing, subject to any other conditions that may be specified.

In addition to materials specification and property selection are options for specifying the sources of the data. The options for the latter criteria include specifying the name of an author, the name of a journal, the publication year, the publication volume number. The name of the author and the volume number are entered using the keyboard after clicking on the appropriate selection box. Please note that the default value for all entries is "Any". The SQL representation for "Any" is "%%". In all cases except for Publication Volume it was possible to shield the user from such an explicit and potentially confusing representation. There were sound technical reasons which made it unavoidable to display "%%" in the case of Publication Volume. The journal and publication year specifications are made by clicking on the desired entry in the appropriate drop-down selection list.

The criteria selection frame has two buttons across the bottom of the window. The Search button effects a search of WebHTS and the Clear button clears the current search specifications.

A brief display of all records that satisfy the search criteria is presented in the frame below the search criteria frame. The citation number for the retrieved record is linked to the detailed display frame immediately below the brief display. Clicking on the citation number will reveal the full contents of the retrieved record. The full content display includes additional links internal to the display document for easier navigational access to specific property data.

V. GENERAL NOTES

Notation

conventional notation is used whenever possible.

Materials

The manufacturer's name is identified for all commercial materials. For other materials, the manufacturer's name is given as "in-house" or "typical". The term "in- house" is used to designate materials produced in limited quantities for experimental purposes. The term "typical" is used when the reported property values are derived from surveys of studies on various materials that are expected to be similar.

Properties

Properties are named according to common usage. When more than one common name may be possible, the most inclusive name is selected. For example, "Young's modulus" is included in this database as an "elastic modulus". When a common name has a significant potential for ambiguity, an alternate name is used. For example, the Weibull analysis of flexural strength data involves a parameter, with the dimensions of strength, that is often called the "characteristic strength". However, the quantity most commonly cited as being characteristic of the strength of the material is the "flexural strength" which is usually given as the arithmetic average of a set of values. To avoid confusion, the name "Weibull strength" is used instead of "characteristic strength".

Property values have been expressed in appropriate SI units. For consistency, all non- SI units in cited quotations are also converted to SI units and represented in the quotation as (SI value = non-SI value); for example, the quotation "in a field of 10 G" would be expressed as "in a field of (1.0 mT = 10 G)".

When a temperature is originally reported as "room temperature", the value has been interpreted to mean 296 K or 23 ºC, as appropriate.

Uncertainties in measured values are expressed in one of two forms, according to the prevalent convention in the field of the measured property. Most commonly, values are expressed in the form xñu which indicates that the value of the quantity is expected to be in the range x-u to x+u. Most commonly, the value of the uncertainty, u, is equal to one standard deviation of a set of measured values. An alternate form, x(u), is used to express uncertainties in crystallographic data. In this form, the value of u indicates the uncertainty in the last reported digit of the value of x. For example, 3.1234(2) is equivalent to 3.1234±0.0002.

VI. CONCLUSION

The user's interface to the WebHTS database is designed to allow the user to search for data without prior knowledge of query methodology and without the necessity of complicated logistical gymnastics. The result is an easy and efficient means of accessing the arduously established evaluated data for high-temperature superconductors.

VII. REFERENCES

[1] M. B. Maple, "High Tc Oxide Superconductors," MRS Bulletin, Vol. XIV, No. 1, pp. 20-21 (1989).
[2] D. K. Finnemore, "Superconducting Materials," Encyclopedia of Physics, Second Edition, R. G. Lerner and G. L. Trigg, editors (VCH Publishers, Inc. 1991) pp. 1190-1193.
[3] H. K. Onnes, "On the sudden change in the rate at which the resistance of mercury disappears," Communication No. 124c, November 1911.
[4] J. R. Gavaler, "Superconductivity in Nb-Ge Films above 22 K," Applied Physics Letters, Vol. 23, No. 8, pp. 480-482 (1973).
[5] J. Bardeen, L. N. Cooper, and J. R. Schrieffer, "Theory of Superconductivity," Physical Review, Vol. 108, pp. 1175-1204 (1957).
[6] J. G. Bednorz and K. A. Muller, "Possible High Tc Superconductivity in the Ba-La- Cu-O System," Z. Phys. B: Condensed Matter, Vol. 64, pp. 189-193 (1986).
[7] Committee on Science, Engineering, and Public Policy, Report of the Research Briefing Panel on High-Temperature Superconductivity (National Academy Press, Washington, D.C. 1987).
[8] R. G. Munro, F. Y. Hwang, and C. R. Hubbard, "The Structural Ceramics Database: Technical Foundations," Journal of Research of the National Institute of Standards and Technology, Vol. 94, Number 1, pp. 37-47 (1989).