01 April 2003

Phosphor thermometry

The essential features are in place for realizing a temperature scale in terms of atomic quantities.

By W. Allison, M.R. Cates, and G.T. Gillies

When it comes to measuring temperature, industry depends on primary, secondary, and transfer-standard thermometers to calibrate instruments.

Terminology

Atomic clock
A precision clock that uses the natural vibration frequency of an atomic system such as a beam of cesium atoms as the clock's operational device.

Doping
Adding an impurity, dopant, to a pure substance or element that changes the properties of that element in a desired manner.

Fluorescence
The luminescence that occurs after a material absorbs energy and goes to a higher energy level. When the material returns to its original energy state, it gives off the energy in the form of light.

Thermographic
Made of ceramic or similar materials phosphors that are doped, or activated, with rare-earth elements. They deposit in thin films on an object's surface. The signal emitted from a thermo graphic phosphor film collects optically—no contact with the object occurs.

Triple point
The temperature and pressure at which the solid, liquid, and vapor phases of a substance are in equilibrium.

But uncertainty is a fact of life in temperature measurement and control. And researchers are now proposing a new technique to improve thermal standards.

Standards laboratories worldwide currently use the International Temperature Scale of 1990 (ITS-90). But to use this scale, they depend on a wide variety of material artifacts, devices, and algorithms.

This temperature scale exists between the values of 13 kelvin (K) and 1,234 K in terms of the well-defined thermodynamic transition points—melting, freezing, vapor pressure, and triple points—of several pure materials.

These elemental materials are the gases (at ambient conditions) hydrogen, neon, oxygen, and argon and the metals (ambient . . .) mercury, gallium, indium, tin, zinc, aluminum, and silver, as well as with water.

The base unit of the International System (SI) associated with the measurement of thermodynamic temperature, the kelvin, defines as 1/273.16 of the thermodynamic temperature of the triple point of water.

Interpolation over subranges of the ITS-90 span shows that there are calibration errors on the order of a few tenths of a millikelvin propagating from the standard.

Physicists, chemists, and metrologists at national standards laboratories around the world contribute to the structure, maintenance, and improvement of the ITS-90. Their goal is to lower overall uncertainty in the realization of the kelvin via this scale.

Independent atomic quantities

Because the material artifacts in use have no direct tie to atomic quantities, precision varies depending on the individual artifact.

Ideally, all of the base units of the SI system would manifest in terms of atomic quantities that are independent of artifacts. With this independence, all laboratories could establish the units with essentially equal precision.

With this purpose in mind, researchers have begun exploring the possibility of using single crystals of thermographic phosphors in a new realization of the kelvin.

The experimental technique would involve the laser-induced fluorescence of a rare-earth-doped phosphor—the optical decay lifetime of which would be measured relative to an atomic clock—in order to obtain a primary standard of temperature based only on atomic quantities.

In principle, this technique could provide the first realization of the kelvin over continuous ranges of temperature, without the need for interpolation between fixed thermodynamic transition points.

And ultimately, this technique could establish a new kind of temperature scale tied directly to atomic constants from which the kelvin could arise and subsequently disseminate with improved precision.

Phosphor thermography measurement system

Change-transfer states

Researchers have explored remote thermometry via thermographic phosphors in depth during the past 20 or so years, and a large number of applications for this methodology have been proposed.

The existence of charge-transfer states in materials such as Y₂O₃: Eu (Yttrium oxide doped with Europium) and La₂O₂S: Eu (Lanthanum oxysulfide: Europium) provides for a temperature dependence in the amplitudes and exponential decay lifetimes of laser-induced fluorescence in such materials.

The excitation energy of these shares between radiative (photon) and nonradiative (phonon) deexcitation processes.

The physics underlying the temperature dependence of the ratio between these deexcitation mechanisms provides useful models for the quantum mechanics of the decay rates.

If satisfactory predictions of temperature vs. decay rate can be made for an appropriate material, and if the subsequent experimentally measured decay rates could be determined using an apparatus that has a cesium beam atomic clock as its time base, then the essential features are in place for realizing a temperature scale in terms of atomic quantities.

The components for accomplishing this type of measurement are several. A tunable dye laser or some other suitable excitation source serves to optically pump the phosphor sample, which is located inside an ideally gradient-free furnace.

The fluorescence signal registers on a fast, low-noise photodetector, and a waveform-processing oscilloscope and/or an offline computer analyzes the signal and extracts the exponential decay time constant.

The time base for the whole system would be a cesium beam atomic clock or possibly a suitable stable signal derived from one.

Given a satisfactory theoretical understanding of the relationship between the temperature of the single-crystal phosphor sample and the associated fluorescence decay time, the resulting measurements will yield the thermodynamic temperature of the sample over the entire response range of the particular phosphor being used.

A thermometer in thermal equilibrium with the sample, such as a platinum resistance thermometer, can then calibrate directly against this new type of primary standard.

Realization of the kelvin over continuous spans of several hundred degrees would then be possible, as some phosphors have slow variations of decay time with temperature.

Prototype experiments

The authors carried out a series of experiments aimed at establishing a thermal measurement chain consistent with this model. The original purpose of the studies was to use the system in the inverse sense and calibrate the response of the phosphor thermometer in terms of a National Institute of Standards and Technology (NIST)–traceable reference thermometer.

They later identified the sources of potential experimental uncertainty in such an experimental arrangement.

One of the principal findings of this work was that one could obtain very pure fluorescence decays from materials such as Y₂O₃: Eu following the initial photooptical charging of the phosphor.

With the availability of samples of single-crystal phosphors suitable for experimental evaluation (LaPO₄: Eu and Y₂O₃: Eu), the authors then carried out sets of studies aimed at demonstrating the feasibility of realizing the kelvin with this approach.

Using the phosphor LaPO₄: Eu, continuous measurements (rather than interpolations) between the freezing points of Zn and Al would be possible. A phosphor with a broader thermal roll-off or lower quenching temperature would be necessary to reach lower temperatures.

Phase diagram for water

Prospects for implementation

The realization of a new primary standard of thermodynamic temperature would be a very ambitious undertaking and would ultimately require the resolution of many important metrological issues.

The conversion of the temperature measurement into one of obtaining and analyzing a time interval, however, moves the concept in a direction that is generally aligned with modern progress in metrology because time and frequency are among the most precisely measured of all variables.

To fully realize the possibilities suggested by this approach, an accurate quantum mechanical model of the temperature dependence of the phosphor's decay time is necessary. Efforts aimed at developing such models are afoot, and further work on this topic is under way.

To make such a system readily available to industrial as well as standards laboratories, there is a need for inexpensive alternatives to laser-based excitation of the phosphor.

The authors' work has demonstrated that blue LED excitation of rare-earth-doped phosphors is possible, thus providing one such alternative excitation source. Lastly, phosphors with a temperature-dependent fluorescence decay time that covers as wide a temperature span as possible should be developed and used.

The authors are now studying both single-doped and double-doped phosphors that show promise of satisfactory performance. Eventually, long-term experiments will be required to establish the durability of the fluorescent material for extended operation at high temperature and over many high/low temperature cycles. TT

Behind the byline

W. Allison and M. R. Cates are senior development staff members at Oak Ridge National Laboratory. G. T. Gillies is a research professor at the University of Virginia. Matthew Lamoreaux contributed to this article. Refer to the original ISA technical paper for discussion and test results graphs at www.isa.org/intech/phosphors.