Since the inception of the SI system of units, physical mass artifacts have been transferred and exchanged in order to disseminate the kilogram. The kilogram is essential to a broad range of measurements in the marketplace ranging from weight to electrical energy. The practice of disseminating the kilogram relies on a multi-tiered comparison of mass artifacts (physical objects). The uppermost tier contains a single "Le Grand Kilogram" (IPK) that is currently what defines the kilogram. Lower tiers are used to disseminate the kilogram to various National Measurement Institutes (NMIs), then to their working standards, etc. In 2018 IPK will be retired and mass units will then be realized from Planck's constant by way of the Watt Balance or by the X-ray-crystal-density (XRCD) method. However, even with this new primary realization, artifacts in the dissemination chains will vary due to their environmental history. To better predict these variations, we are studying how the mass of different materials changes with controlled exposures to environmental gasses at different temperatures. Our strategy uses an ultra-sensitive resonator based micro-weighing method that has sub-monolayer sensitivity to correlate mass changes with loading and unloading of environmental gasses.
We have implemented a mechanical double paddle oscillator (DPO) as a micro-weighing sensor and monitor a high quality factor resonance. The resonance frequency varies proportionally with the mass loaded on the oscillator. A photograph of the DPO sensor assembly is shown in Figure 1, as mounted on the cryo-cooler stage that allows the temperature of the assembly to be varied from 4 K – 500 K. In this implementation, we are able to perform measurements using inert gas ices (e.g., Ne, Ar, Kr, Xe, N2, etc.) and benchmark the loading and unloading performance against high precision measurements already available in the literature. The DPO and cryo-cooler are inserted into a UHV vacuum chamber that is equipped with multi-gas leak and composition monitoring capability.
An example of loading the DPO with nitrogen ice at 8.9 K is shown in Figure 2. When nitrogen gas is first introduced the chamber pressure rises and the DPO frequency drops. As the N2 gas collects as nitrogen ice on the DPO, the DPO's total mass increases causing the resonance frequency to decrease. The growth of the ice is stopped by closing the nitrogen source valve and pumping out the excess gas. We then increase the temperature of the DPO in slow, discrete steps and monitor the rate of mass (frequency) change. After measuring sublimation rate at different temperatures we can determine the enthalpy of sublimation. These measurements are a precursor to enthalpy of reaction measurements that are more relevant to mass changes on existing mass artifacts.
For environmentally driven mass changes, we plan to deposit films (e.g. aluminum, gold, etc.) using materials similar to mass artifacts or proposed coatings. The experiments will each start with a bare DPO onto which we will deposit the solid film in situ. The unreacted surfaces will then be exposed to gasses while we monitor in real time mass change due to reaction, uptake and/or release. Comparisons between materials or coatings can then be used to stability of the system for use in mass dissemination.
Additionally, this apparatus is able to be equipped with a commercial quartz crystal monitor (QCM) for inter-comparison between the DPO and QCM. Using the QCM in combination with the DPO and ex situ mass comparisons are being evaluated as a means for establishing a NIST traceability link for QCM measurements.