Performance of micro-balances for dust flux measurement

A&J. Space Rex. Vol. 29, No. 8. pp. 1155-l 158,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Pergamon Printed in Great Britain 0273-I 177/02 $22.00 + 0.00 www.elsevier.com/locate/asr PII: SO273-1177(02)00131-X PERFORMANCE OF MICRO-BALANCES MEASUREMENT FOR DUST FLUX E. Palomba”2, L. Colangeli2, P. Palumbo3, A. Rotundi3, J. M. Perrin2, E. Bussoletti3 ‘Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy 2 Observatoire de Haute Provence du CNRS, F-04870 Saint Michel l’observatoire, France ‘Istituto di Matematica, Fisica e Applicazioni, Istituto Universitario Navale, Via De Gasperi 5, 80133 Napoli, Italy ABSTRACT Micro-balances have been used in the past for volatile deposition monitoring in laboratory and in space environment. In order to determine their suitability to measure mass deposition in the form of solid particles, some topical aspects must be characterised, such as the sensitivity versus temperature and grain mass and the sticking efficiency versus grain speed. These parameters have been retrieved for different sensor configurations, i.e. with and without an adhesive coating, used in the perspective of improving the sensor particle collection efficiency. Our studies show that the adhesive coating improves the sensor sticking efficiency only for fast (100-400 m se’) grains. However, the stability of the output signal with temperature is worse in the coated configuration by a factor of about ten. These results provide important inputs in the view of using micro-balances for dust monitoring. In particular, they have been carefully considered for the selection of the configuration of micro-balances, included as sub-systems of the GIADA experiment onboard the ESA ROSETTA mission and aimed at studying flux and dynamic properties of cometary grains. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Micro-balances have been often used in the past for analysing the deposition of films in space (e.g. Kent et al., 1994). Their use for monitoring dust collection requires a careful analysis of performances vs. dust physical and dynamical characteristics. Micro-balances usually consist of two oscillating quartz crystals, the sensor and the reference, resonating at a frequency of lo-25 MHz. We recall that the sensitive part of the crystal is its electrode (i.e. a thin gold coating). The two crystals are chosen with slightly different (some kHz) resonance frequencies to have f, > f,, where f, and f, are the reference and sensor frequencies, respectively. The output signal is the beating frequency between the two crystals f = f, - f,. When mass is added to the sensing crystal, f, decreases while f, remains constant, so that f becomes larger. The sensitivity of the micro-balance (in Hz g-i) determines the link between collected amount of mass and frequency change. Saturation of the sensor occurs when the beating frequency reaches about 1% of the resonant frequency. In this paper we present the results obtained by studying the behaviour of a micro-balance (Mod. MK2 1, provided by QCM Research, Laguna Beach California), resonating at a frequency of about 15 MHz, in the perspective of using them to collect solid grains. The nominal sensitivity-’ of the micro-balance is 0.2 ng Hz-’ and refers to the deposition of mass in the form of a uniform film. Saturation of the sensor is expected at about 10’ kHz. For applications to dust it is necessary to determine the sensor sensitivity for particulate deposition. Another important question to be addressed concerns the capability to capture grains impinging (at a given velocity) on the micro-balance: the grains will be detected only if they stick onto the sensor crystal. Therefore, it is important to determine a sticking function as the ratio between impacting vs. collected grains. Last but not least, the sensor stability vs. temperature is another important parameter to be determined. These analyses are fundamental in the perspective of the GIADA project. The GIADA (Grain Impact Analyser and Dust Accumulator) experiment (see Bussoletti et al., 1999 for more details), onboard the ESA ROSETTA mission (Bar Nun et al., 1993; Schwehm. and Schulz, 1999) is devoted to study cometary dust dynamics and time evolution, during about 1 year of orbiting around comet 46PWirtanen. Among the different devices forming GIADA, the Micro-Balance System (MBS) will allow the detection of the cumulative dust deposition in time. The 1155 E. Palomba et al. 1156 MBS consists of five quartz micro-balances, similar to those analysed here, pointing towards the comet nucleus and four other directions. The cometary grain velocities, expected at the ROSETTA probe, are in the range of about l-400 m s-l (Fulle et al, 1999), depending on the grain size. Therefore, in our laboratory tests, we focused on such grain velocities. Moreover, in the study of the crystal sticking efficiency, we considered as a possible improvement the use of an adhesive coating. Different materials were considered and their properties were studied. The Apiezon L vacuum grease has been already used as coating of micro-balances for aerosol monitor. It has a very low vapour pressure (lo-” Torr @ 20°C), optimal for space applications. For this reason we used this material for our laboratory investigations. Other promising materials appear some polymers such the ones tested for the dust collector of the MIDAS instrument onboard Rosetta (J. Romstedt et al., 1999). Its use in couple with micro-balance sensors will be subject of future investigation. We recall that the presence of a coating could influence the properties of the microbalance, such as the stability vs. temperature of the output signal and the sensitivity. These aspects were carefully analysed in our experiments. In the following sections we will report the main results obtained from our analysis on sensor stability vs. temperature, sensitivity and sticking efficiency with and without Apiezon L coating. The conclusions drawn as a follow up of our analyses demonstrate that “clean” micro-balances appear the best solution for dust collection, at least in the experimental configuration expected during the ROSETTA cometary mission. SENSOR STABILITY The nominal sensor stability of the MK21 micro-balance is 1 Hz ‘C’. In our experiment, to determine the stability of the system, for both the coated and the uncoated configurations, we measured the sensor output signal vs. temperature in the range -20 “C + +6O”C. The maximum retrieved gradient for the uncoated sensor is about 1.9 HzPC in the range (-17 t +57) “C, consistent with the specifications. The situation drastically changes for the coated configuration, for which a maximum gradient of 66 Hz ‘C’ is retrieved in the hotter range (34 + 47) “C. In the interval (-17 + 34) “C the measured gradients are of the order of 6-7 Hz “C-l. These results indicate that the coated sensor is less stable than the uncoated one and its use in space could be critical. SENSOR SENSITIVITY To measure the micro-balance sensitivity to solid grains, four grain sets were used: amorphous submicrometer forsterite and carbon grains, 1.2 pm Si spheres and 10 pm Si spheres. In order to study the microbalance sensitivity to solid particles, we released calibrated amounts of de-agglomerated grains onto the crystal electrode. To this aim, we dissolved the grains in ethanol by shaking the solution in an ultrasonic bath. The solid material in the solution was calibrated in mass in order to deposit fixed quantities of dust. After the ethanol evaporation, we could measure the changing in frequency due to a known amount of solid mass released onto the micro-balance crystal. For each run we released about 1 ug of dust onto the crystal electrode. Also for this test we used both coated and uncoated electrodes. The results are reported in Table 1. For the sub-micrometer amorphous grains the micro-balance sensitivity to solid grains is close to the nominal value, both for the coated and the uncoated sensor configurations. The situation changes for spherical grains: the sensitivity-’ is larger of a factor of 45 with respect to the nominal value for 1.2 j.trn particles (uncoated configuration), while for the 10 pm spheres the micro-balance is insensitive, both for the coated and the uncoated configurations. These results seem to imply a size dependence of the micro-balance sensitivity: smaller grains are detected more efficiently by the sensor. We cannot exclude a dependence on morphology because measurements of spherical and irregular grains of comparable size are not available at this stage. We can, however, conclude that for sub-micrometer grains the sensitivity of the micro-balance is not significantly affected by the presence of the coating. The MBS-Giada System for the Rosetta Mission Nominal sensitivity’ 0.2 ng Hz-’ Uncoated Coated Table 1 - Micro-balance microns in thickness. STICKING QCM MK2 1 Sensitivity’ 1157 (ng Hz-‘) 1.2 urn Si spheres Carbon Forsterite 0.91 f 0.23 0.27 f 0.04 0.52 f 0.09 0.23 f 0.04 0.29 f 0.05 sensitivity to solid grains. Different grain sets were analysed. The coating is some STUDIES The sticking function studies are described in detail elsewhere (Palomba et al., 2000). Here we recall the most important results concerning the evaluation and the comparison of the sticking functions for coated and uncoated micro-balance crystals. According to the results of the sensitivity studies, the expected velocities for the micrometer and submicrometer grain sizes, i.e. the sizes detectable by the GIADA-MBS, are in the interval lo-100 m se’ (Fulle et al, 1999). We focused our efforts on this grain velocity ranges. Low velocities regimes (< 55 m s“) This experiment was performed by using a system able to shoot solid micrometer particles with velocity up to 90 m s-‘. With this set-up we were able to follow the single particle trajectory, to calculate its pre-impacting velocity (Poppe et al., 2000) and to discriminate its bouncing or capture by the target. We used as projectiles two grain samples: 1.2 urn Si microspheres and 0.64 pm SIC irregular grains. For spherical grains we found that the coated electrode is stickier than the uncoated one. In fact, the capture velocity, i.e. the velocity value for which the sticking efficiency is equal to 50%, is double in the presence of the Apiezon L coating. However, for both coated and uncoated configurations, the sticking efficiency is nearly zero for grain velocities larger than 10 m se’. The situation is different for irregular grains, as the sticking efficiency for the uncoated electrode is very high (about 80%) up to grain velocities of about 20 m se’. We can conclude that the presence of the coating does not augment the sticking efficiency for velocities smaller than 20 m se’. For higher grain velocities and up to 55 m s-’ we could not follow the single trajectories, but we could infer the sticking efficiency of the uncoated relative to the coated electrode. This was obtained by bombarding simultaneously the two samples by the same dust flux and afterwards by counting the solid particles deposited on the surfaces of the samples. The relative sticking function, obtained by dividing the number of grains observed per unit area by a scanning electron microscope onto the two samples, was calculated to be 0.8. It is, then, reasonable to state that, in the entire velocity range I - 55 m s-‘, the sticking efficiency of the electrode is very high and the use of an adhesive coating does not improve substantially its grain collection efficiency. High velocities regimes (200-400 m s-l) To extend the previous experiment towards higher grain velocities, we used a dust accelerator to shoot 0.64 urn Sic grains at velocities of 200-400 m s-‘, thanks to the collaboration with Prof. F. Riidenauer in the Austrian Research Centre in Vienna. In order to evaluate the collection efficiency we adopted the method of the particle counting, as explained above. The relative (uncoated vs. coated electrode) sticking efficiency was found to be less than 0.1. This means that in this velocity regime the presence of an adhesive coating improves the grain collection efficiency of the micro-balance sensor. 1158 E. Palomba zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH et al. CONCLUSIONS The coated configuration is strongly sensitive to thermal variations, especially for T > 35 “C (melting point of the Apiezon grease), while the uncoated one is much more stable. The sensitivity-’ to sub-micrometer irregular grains, with and without the coating, is close to the nominal value, whereas for spherical micrometer grain is five times smaller. The tests performed revealed that the sensor is not able to detect the 10 pm spherical grains, both in the coated and in the uncoated configuration. The sticking efficiency of the uncoated sensor is very high for low velocity particles but decreases of a factor of about 10 with respect to the coated configuration for velocities larger than about 200 m s-i. Considering all these results it is recommended to adopt the uncoated configuration for the micro-balances to be used in the GIADA-MBS. In this configuration the GIADA-MBS is able to detect submicrometer to micrometer cometary grains, with a well-constrained stability. The detectable size range is complementary with the one measurable by the other GIADA sub-systems (Impact Sensor and Grain Detection System). This makes the performances of the GIADA instrument compatible with its goals, allowing the monitoring of the dust flux emitted by the comet 46PWirtanen in a wide size range. ACKNOWLEDGMENTS We thank E. Zona, N. Staiano and S. Inarta, for technical assistance during experiment execution. We warmly thank Prof. F. Rtidenauer in the Austrian Research Centre, Seibersdorf, Vienna, Austria, and Dr. T. Poppe in Jena for their collaboration in the implementation of the sticking studies. We thank QCM Research for providing their quartz crystals in a collaborative research within the GIADA project. Work supported by ASI, MURST, CNRS. REFERENCES ESA Bar-Nun A., Barucci A., Bussoletti E., Coradini A, Coradini M. et al., cComet Ro a SCI(93) 7, 1993. Bussoletti E., Colangeli L., Lopez Moreno J. J., Epifani E., Mennella V. et al., The GIADA experiment for Rosetta mission to comet 46PlWirtanen: design and performances, Adv. Space Res., 249, 1999. Fulle, M.; Crifo, J. F.; Rodionov, A. 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