Reference materials

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High temperature pushrod dilatometer (Austrian Foundry Research Institute, Leoben)

A reference material is a material for which one or more of its property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus; assessment of a measurement method; or for assigning property values to other materials. ( The International Vocabulary of Basic and General Terms in Metrology , or VIM, 1993 ).

Reference materials are carefully measured and usually certified materials (see Definitions at NIST for the different kinds of reference material). In the thermal field they are an essential tool for verification and use of instruments that measure thermal properties such as thermal conductivity, diffusivity, expansivity, emissivity, enthalpy and melting point, among others.


High-speed double-Michelson-interferometer for metallic materials (Austrian Foundry Research Institute, Leoben)

      
Reference materials may be used to calibrate measurement instruments, to compare, develop and validate methods and to establish traceability of measurements, i.e. to ensure comparable measurement results between different laboratories. Present standards for the accreditation of testing and calibration laboratories (such as the international standard EN ISO/IEC 17025) stipulate the use of reference materials.

Reference materials must be homogeneous and stable, and the certified values must be accurate. Finding a material that can be used as a reference for more than one property is one of the goals of current research.

Thermal expansion is measured with highest accuracy by interferometry or usually with somewhat lower accuracy by pushrod dilatometry.

For interferometric measurements, the thermal expansion is derived from the counting of the shifting fringes, only the monochromatic laser wavelength has to be known. Thermal expansion standards are only used for verification of the interferometer.

Pushrod dilatometers have an instrument characteristic (the specimen holder itself expands too), which has to be compensated for. This instrument characteristic depends among others mainly on the specimen holder material (it should have a small thermal expansion coefficient in the entire temperature range of operation) and the heating/cooling rate. That means, all pushrod dilatometers having either one or two pushrods measure relative to a reference material with known thermal expansion as a function of temperature.

     


In-situ temperature calibration of a dilatometer: specimen holder with pushrod, thermocouple and gold droplet, the gold specimen collapses at the well defined melting temperature

In-situ calibration of the temperature or checking for temperature gradients in the specimen holder of a dilatometer is done by measuring a material with well known phase transformation or melting temperature (eg pure elements). 

Reference materials are supplied by international organisations (click here for the IRMM catalogue), research institutes (e.g. NIST, NPL, PTB, NMIJ) or by manufacturers and suppliers of instrumentation (e.g. Netzsch, Linseis).

In the table below is a selection of reference materials for thermal expansion. A fuller list of reference materials for this and other properties, and their suppliers, is available through a search of Reference materials.

Reference materials for thermal expansion

Reference material

Property

Notes

Link

Borosilicate Glass

Thermal expansion

Low thermal expansion; temperature range limited (80-680 K), certified thermal expansion values for individual specimens

NIST

Copper

Thermal expansion

Temperature range 20-800 K, certified thermal expansion values for individual specimens

NIST

Stainless Steel (AISI 446)

Thermal expansion

Temperature range 293-780 K, certified thermal expansion values for individual specimens

NIST

Platinum 99.999+%

Thermal expansion

DIN 51045 standard recommends thermal expansion values and uncertainties but no suppliers are listed

DIN

Vitreous silica

Thermal expansion

DIN 51045 standard recommends thermal expansion values and uncertainties but no suppliers are listed

DIN

Corundum single crystal

Thermal expansion

DIN 51045 standard recommends thermal expansion values and uncertainties but no suppliers are listed.
High temperature (up to 1773 K)

DIN

Alumina polycrystalline

Thermal expansion

DIN 51045 standard recommends thermal expansion values and uncertainties but no suppliers are listed.
High temperature (up to 1773 K)

DIN

Copper

Thermal expansion

Recommended thermal expansion values, Int J Thermophysics, Vol 18, 1269-1327 (1997)

CODATA

Silicon

Thermal expansion

Recommended thermal expansion values, Int J Thermophysics, Vol 18, 1269-1327 (1997)

CODATA

Tungsten

Thermal expansion

Recommended thermal expansion values, Int J Thermophysics, Vol 18, 1269-1327 (1997), up to 3500 K

CODATA

Alumina

Thermal expansion

Recommended thermal expansion values, Int J Thermophysics, Vol 18, 1269-1327 (1997)

CODATA


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