HR: 0830h
AN: V41C-0308    [PDF]
TI: An Ultrasonic Frequency Sweep Interferometer For Sound Speed Measurements On Liquids At High Temperature And Pressure
AU: * Ai, Y
EM: yuhui@umich.edu
AF: DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MICHIGAN, ANN ARBOR, 2534 C.C LITTLE BUILDING 425 E.UNIVERSITY AVENUE, ANN ARBOR, MI 48109 United States
AU: Lange, R A
EM: becky@umich.edu
AF: DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MICHIGAN, ANN ARBOR, 2534 C.C LITTLE BUILDING 425 E.UNIVERSITY AVENUE, ANN ARBOR, MI 48109 United States
AB: One of the most direct methods for obtaining melt compressibility is through measurements of sound speed via acoustic interferometry. This technique may be applied to silicate melts by either varying the path length or the frequency of the acoustic wave through the melt. To date, only the variable path length (VPL) technique has been applied, which restricts measurements to atmospheric pressure owing to the requirement of mechanical movement of the upper buffer rod. This, in turn, precludes the study of volatile-bearing liquids at pressure and a systematic study of how melt compressibility varies with pressure. We have developed a frequency sweep (FS) interferometer that can be applied at high pressure, which is based on frequency spectrum analysis on mirror reflection waves from high-temperature liquids. First, a theoretical acoustic model for a rod-liquid-rod (RLR) interferometer is proposed and solutions to the resultant wave equations are obtained. The solutions demonstrate that only two kinds of non-dispersive waves exist within the upper buffer rod. They have computable group velocities and waveform patterns that are entirely dependent on the material and diameter of the buffer rods. Experimental tests verify the theoretical model and indicate that buffer rods made of molybdenum metal and $>$ 1.9 cm diameter are ideal for sound speed measurements in silicate melts with the FS interferometer. On the basis of the theoretical acoustic model, a mechanical assembly and signal-processing algorithm was designed to implement the FS interferometer. A very short pulse (e.g. 1 microsecond) encompassing a range of frequencies that span about 1 MHz is sent down the upper buffer rod and the first two mirror reflections from the liquid are collected and stored. Because they have the same waveform and have 180$^{o}$ phase difference, Fourier spectrum analysis can be performed to find the frequency response function of the two reflections, which is related to the sound speed and thickness of the melt. From the obtained frequency response function, the sound speed is calculated. We have applied this newly designed FS interferometer to two liquids with well-known sound speeds from the literature: NaCl liquid at 930$^{o}$ C and 1026 $^{o}$ C and a sodium aluminosilicate liquid at 1436 $^{o}$ C. Sound speeds were measured for these liquids at three center frequencies (4.5 MHz, 5.0 MHz, and 5.8 MHz). Our results are less than 0.6 % off the literature values and demonstrate the accuracy and precision of the FS interferometer. The principal advantage of the FS interferometer over the VPL method is that it requires no physical intervention or mechanical movement of the micrometer-transducer-rod assembly during a measurement. Thus, the FS method has considerable promise for adaption to high-pressure conditions in an internally-heated pressure vessel.
DE: 3630 Experimental mineralogy and petrology
DE: 3919 Equations of state
DE: 3939 Physical thermodynamics
DE: 3994 Instruments and techniques
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting