Acoustic Measurements and Methods for Thermophysical Properties of Gases

Both the thermophysical and transport properties of gases are important in a broad range of industrial processes ranging from thermo acoustic machine design to measurement of flowrates. Investigation of fundamental physical mechanisms are also based on versatile and rugged acoustic resonator methods designed to produce highly-accurate measurements of the speed-of-sound and viscosity of gases. A modified Greenspan acoustic resonator with specialized transducers is being developed to measure the bulk viscosity of xenon near its critical point in earth's gravity and, eventually, in microgravity.  These measurements will be made closer to the critical point than ever before, and the results should resolve some long-standing discrepancies between theory and experiment. This work complements expertise gained from the previous measurements of the shear viscosity of near-critical xenon in microgravity.

Gas phase chemical delivery is a fundamental strategy in the manufacture of microelectronic devices. Mass flow controllers (MFCs) are ubiquitous for gas delivery to process chambers. Individual process tools use a significant number (20 - 50) of MFCs to control gas delivery, much of which is in the 1 to 1000 sccm flow rate range. Continued increases in process reproducibility requirements drive the need for improvements in MFC accuracy and stability. A recent workshop report identified two areas as critical to the improvement of MFC performance, accurate thermophysical property data for process gases and improved accuracy of transfer standards necessary to provide traceability to the SI. Improvement in flow rate measurement standards in the 1 to 1000 sccm range will be improved by developing more reproducible low flow rate transfer standard technologies and the reduction of the uncertainty in low flow rate primary measurement standards. Property data provides the basis for assignment of gas conversion factor values commonly used to correlate MFC flow performance obtained in calibration tests using non-reactive gases with performance when used with reactive process gases. NIST research is directed toward improved standards in this low flow rate range and the provision of thermophysical and transport property data for chemically reactive process gases with efforts to: (1) develop techniques for measuring the equation of state and transport properties of the gases used in semiconductor processing with the uncertainties required by industry, (2) develop the computational tools necessary for analyzing, evaluating, correlating, and (where possible) predicting data for these properties, and (3) disseminate property data via a user-friendly database and archival publications. As data are acquired, they are posted on the Internet, in tabular form, at http://properties.nist.gov/ semiprop .  The required properties include: speed-of-sound, heat capacity, density (equation of state), viscosity, and thermal conductivity. 

Process Gases

Acoustic resonators were used to determine the speed of sound and the viscosity of ten process gases. These gases are listed below, together with the ranges spanned by the measurements. The results for these 10 gases will be published in archival journals. Two manuscripts are in preparation.

When researchers in the CSTL's Physical and Chemical Properties Division, Boulder were developing a correlation to predict the dielectric constant of natural gas from the composition, temperature, and pressure, they discovered contradictory data for the dielectric constant of CO 2 . At their request, we made new measurements to resolve the contradiction and published the results in:

• May, E.F., Moldover, M.R., Schmidt, J.W., “The Dielectric Permittivity of Saturated Liquid Carbon Dioxide and Propane Measured using Cross Capacitors, ” Intl. J. of Thermophysics (in press).

Fundamental Studies of Transport Properties

We developed a technique for using forced convection to stir near-critical fluid samples. By heating the samples from below, we reduced their density stratification under the earth's gravity by two orders of magnitude. This enabled us to measure the bulk viscosity of xenon 60 times closer to the critical point than previous researchers. Prior to this work, the community believed that these results could only be achieved in the microgravity environment provided by the Space Shuttle or the International Space Station. These results have been accepted for publication:

• Gillis, K.A., Shinder, I.I., and Moldover, M.R., “Bulk Viscosity of Stirred Xenon Near the Critical Point,” Phys. Rev. E (in press).

We corrected an error in a recent publication that claimed to determine the transport properties of near-critical fluids from computer simulations. The simulations were too small and their results were inconsistent with good measurements and with analytical theory.

• Sengers, J.V. and Moldover, M.R., “Comment on Molecular Dynamics Simulations of a Fluid Near its Critical Point,” Phys. Rev. Letters Vol 94, 069601 (2005).

 New Standards of Temperature and Pressure

A collaboration between NIST and the French standards lab BNM-INM used a quasi-spherical cavity as a microwave and as an acoustic resonator to determine the errors in the International Temperature Scale of 1990 (ITS-90). Although preliminary, these results are already the best ever in the temperature range 7 K to 25 K. They have been communicated to the Comité consultatif de thermométrie and they have been incorporated into a manuscript for Metrologia that is now undergoing WERB review.

• Pitre, L., Moldover, M. R. and Tew, W. L., “Acoustic Thermometry: New Results from 273 K to 77 K and Progress towards 4 K” , Metrologia, (in review)

We continue to make progress towards an atomic standard of pressure. During FY05, we learned how to reliably manufacture copper-plated, maraging-steel quasi-spherical microwave resonators for use as a pressure standard. We learned how to measure the resonator's temperature with sub-millikelvin resolution while at pressures up to 7 MPa. We learned how to float a piston gauge under computer control.

• Use acoustic resonators to characterize mixtures of {air + water vapor}. This will lead to better humidity standards (particularly near 100 ° C where automotive fuel cells operate) and to better models for exhaust gases. These measurements will support NIST's development of a data base for moist air.
• Use photoacoustic spectroscopy to monitor the concentration of a mixture of either {HCN + air} or {C 2 H 2 + air} Ultimately, we hope to learn how to quantify dilute gas mixtures in field situations.
• Use two capillary flow standards in series to provide reference values of the zero-density viscosity argon and of methane as a function of the temperature. The uncertainties will be 0.02 %, a factor of 10 advance in the state of the art. We will use the same apparatus with xenon to test the theory for the bound-state contributions to the viscosity
• Measure the speed-of-sound and the bulk viscosity in near-critical xenon as a function of the temperature and density
• Use a quasi-spherical microwave cavity to measure the dielectric permittivity e r of helium in the range 0.1 to 7 MPa. From quantum mechanics and statistical mechanics, the theoretical value of e r for helium is known to a few parts per million. If the largest uncertainty in the measurement results from the uncertainty NIST's primary pressure standards, this apparatus will become a new primary pressure standard.
• Use a quasi-spherical microwave cavity to measure e r of argon in the range 0.1 to 7 MPa, 0 ° C to 50 ° C. This will permit the use of argon as a secondary pressure standard.