DEVELOP A SURFACE ACOUSTIC WAVE INSTRUMENT FOR ATMOSPHERIC AEROSOL AND GAS MEASUREMENTS

A NEW TYPE OF PIEZO-ELECTRIC CRYSTAL, THE SURFACE-ACOUSTIC-WAVE DEVICE (SAW), SHOWS PROMISE OF OFFERING MASS-MEASURING SENSITIVITY TWO ORDERS OF MAGNITUDE GREATER THAN THAT OF THE CONVENTIONAL QUARTZ-CRYSTAL-MICROBALANCE IN CURRENT USE FOR ATMOSPHERIC AEROSOL RESEARCH. THE SENSITIVITY OF THE PIEZO-ELECTRIC CRYSTAL INCREASES WITH THE SQUARE OF ITS FREQUENCY. THE CONVENTIONAL CRYSTAL WHICH IS DRIVEN IN BULK VIBRATION HAS A PRACTICAL LIMIT OF ABOUT 10 MHZ IN FUNDAMENTAL FREQUENCY, BECAUSE HIGHER FREQUENCY REQUIRES THINNER CRYSTAL, WHICH IS MECHANICAL TOO FRAGILE. IN THE SAW CRYSTAL THE VIBRATION TAKES PLACE ALONG THE SURFACE, AND CAN BE MADE TO REACH 100 MHZ WITHOUT THE PROBLEM OF STRUCTURAL FRAGILITY. THE INCREASED SENSITIVITY WOULD MAKE IT POSSIBLE TO MEASURE VERY FINE PARTICLES (LESS THAN ABOUT 0.1 M) AND TO MEASURE TRACE GASES BY USING CHEMICALLY REACTIVE COATINGS ON THE CRYSTAL. SUCCESSFUL APPLICATION OF THE SAW TECHNOLOGY COULD LEAD TO VERY SENSITIVE MASS SENSOR FOR ATMOSPHERIC AEROSOL AS WELL AS COMETARY DUST STUDIES BOTH IN THE ATMOSPHERE AND IN SPACE.