Thermal Lattice Expansion as a Tool for Temperature Validation in Non-ambient XRD
While non-ambient attachments for X-ray diffraction (XRD) are usually equipped with precise temperature sensors, there is always a remaining uncertainty in the sample temperature due to the unpredictable properties of the sample material and environment. This challenge can be overcome by temperature validation. In this report, two methods of temperature validation are introduced, and the validation process using thermal lattice expansion is explained in detail.
Why is temperature validation needed?
The two most common temperature sensor technologies employed in non-ambient XRD attachments are re-sistance temperature detectors (RTDs) and thermocou-ples. While both technologies differ in the physical principal they are based on and come in many different variations, both share the benefits of high reproducibility, stability, and accuracy. The readout value of the sensors is usually within fractions of a degree Celsius of the temperature that is actually present at the sensor. Unfortunately, this introduces one of the biggest challenges for the design of non-ambient XRD attachments: How can one be sure that the temperature at the sensor is the same as the temperature at the surface of the sample material, which is what is usually investigated by XRD?
There is an unavoidable offset between the readout tem-perature of the sensor and the actual sample of the sample. This offset is influenced by many different factors such as: the location and proximity of the sensor relative to the sample, the thermal conductivity and thermal contact of the sample and whatever medium is between the sample and the holder, the sample properties such as emissivity, particle size, density and color, the gas atmosphere inside the attachment, and the design of the heater utilized in the attachment. While the offset cannot be avoided and is hard to predict, it can be measured. With the results of these measurements, the readout value can then be corrected to match the temperature at the sample surface precisely. This method is called temperature validation.
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