Reference materials for thermal transport properties measurements

Accurate knowledge of thermal properties, particularly thermal conductivity and its allied properties, is essential for successful modeling, design, operation and economics of any process or system involving transmission of heat. In theory, the measurement of thermal conductivity is often viewed as a simple task but in practice it proves more complex and difficult, particularly when required now to have acceptable high precision levels over a broad range of temperature, above and below room ambient. For such purposes, depending on the type and form of specimen, the thermal conductivity (conductance) of a material, composite or system, covers an approximate very broad range of six orders of magnitude. Consequently, numerous different standard test methods have been developed, and their adequate verification is highly dependent on the use of reference materials. The present paper is based on a previous comprehensive review (Tye in Invited lectures. In: Gaal DS, Gaal PS (eds) Thermal conductivity 30 thermal expansion 18, chap 2. DEStech Publications Inc, Lancaster, PA, 2010) presented to a specialist audience at a conference having direct interests in quantitative thermal property measurements. It contained detailed up-to-date information, on the necessary requirements of test methods, documents and a discussion on the development, and use of the limited number and various types and forms of reference materials (artifacts), then available to support the measurements. Since that time, there has been a significant increase in the use of various newer methods, devoted especially to small specimens; however, little or no work has been carried out to increase the numbers and types of reference materials required for their verification. Newer techniques include one using a modification of the transient plane source technique, used by many of the perceived present audience having similar but differing requirements in qualitative measurement. The present discussion now concentrates more on these developments, availability and use and the need for additional reference materials. Specific emphasis is placed on the driving forces and difficulties involved in assembling a solution for the many current and future requirements. Differences in the type and use of available reference materials are described with examples, together with illustrations both of their correct use and misuse, or abuse that has, or may occur. Applications, where requirements for new standards are considered, and suggestions made to undertake a comprehensive study to develop additional relevant references and to improve the efficacy and precision of the newer methods.