Payload design for microgravity science in the context of space materials research in Mexico

0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR

Adv. Space Res. Vol. 8, No. 12, pp. (12)163—(12)166, 1988 Printed in Great Britain. All rights reserved.

PAYLOAD DESIGN FOR MICROGRAVITY SCIENCE IN THE CONTEXT OF SPACE MATERIALS RESEARCH IN MEXICO R. Pera1ta~Fabi*and F. J. Mendieta~Jiménez** *Instjtuto de IngenierIa-GIAEIUNAM, A.Postal 70—472, Coyoacán 04510, D.F. Mexico * *Instituto de Investigaciones Eléctricas, Palmira, Mor. Mexico

ABSTRACT

A description of a payload prepared and readied for launch in the Space Shuttle’s GAS/CAN programme in March 1986 is presented. Alloy solidification and a thin film growth experiments are described in some detail, together with the in-situ measurement techniques that are used to monitor and control the test parameters. Infrared remote sensors are utilized to follow and control the temperature sequences of the Zn-Al-Cu alloy experiment. A cold cathode vacuum sensor is used in connection with the thin film growth, which is effectuated, via vapor phase deposition, over several mono and polycrystaline samples, with an Al source. The experiments described are a part of a general programme in which microgravity research is conducted at the National University of Mexico. INTRODUCTION Microgravity materials science is gradually becoming a most important activity in earth orbit, in view of the effects allowed by the unique experimental conditions of space. Several phenomena of interest: fluid phase behaviour and solidification front propagation have been observed and quantified, see for example [1-3]. Other work, such as dendritic growth [4], reduced sedimentation and convection [5] and particle dispersion [6], has also opened new lines of research. With regards to epitaxial crystal growth, higher microstructural homogeneity, most likely at a cost of a reduced sample size, but with better electronic properties have been obtained [7]. Solid to solid interfaces is also a field of interest, because the ample technical implications of their behaviour and the not so well known effects associated to the processes of initial growth. The present state of the art ensures a rapid growth in the comming decades, demonstrating the need to conduct quite diverse studies in space processing of materials. In this context, and with the addition of new topics every year, such as biomaterials, the groups working in microgravity research at the university (UNAM), have initiated within the last four years, a programme of experiment selection and payload integration aimed at developing first hand knowledge in the field. EXPERIMENTAL ARRANGEMENT The experiments described in this paper are designed to comply with the Small Self-contained Payload programme in NASA/GSFC, based in the Shuttle [8], also refered to as the GAS/CAN programme. In a previous publication [9], the authors have presented a specialized description of the hardware, this paper refers more specifically to the in-situ measurement techniques, as well as to the data validation analysis, and scientific objectives. The payload is contained in a sealed canister (dia. 50cm x 70cm> includes all of the elements necessary for an automatic realization.

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R. Peralta-Fabi and F. J. Mendieta-Jiménez

Fig 1 shows a general arrangement of several experiments in the mass of 90kg. The chamber for the container, that are limited to a vapor deposition experiment is vented to space, and a vacuum better than 10’ Torr is expected, depending on the orientation of the velocity vector with respect to the purge orifices. A cold cathode vacuum gage is used to measure and control the experimental parameters in flight. This type of device was selected in view of: rugged design, no filament that can b~irnor 9Torr); brake; measurement range electrical (10 —~ lO leakage safe operation, is prevented by the presence of insulation between anode and cathode; and the absence of sputtering during operation. The solid state controller of the gage is operated by the microcomputers on board. The chamber is composed by means of W baskets, batteries controlled by crystals onto which the

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evaporation chamber —batteries and control

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Figure 1. Disposition of hardware within the SSCP canister.

of three independent lobes, where Al is evaporated that are powered by pulse trains comming from Mosfet transistors. Every lobe houses a set of Al is deposited, see fig. 2.

Their operation is independent as to the sequence of events or the failure of any one component. Two sets of identical samples are equiped with heaters, for treatment during or after evaporation. The temperature of the samples

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(monocrystaline: Si, SiO 2’ Cu, dNa, Ag and GaAs; as well as polycrystaline Cu, Si and Ag) may reach and hold 300°C during a predetermined time period.

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chamber body The alloy solidification experiment sample holder consists of a graphite oven heated and heater by an external Ni Cr resistance, under control the of several Figure 2. Diagram of metal evaporation independent loops, see fig. 3. chamber. Temperature measurements are performed by thermocouples and an infrared teledetector that focuses black body radiation of the heated sample onto a PbSe detector, chosen in view of its appropriate spectral response in the 4 —~ 6 ~ region. thermal insulation ~~ses

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sample graphite oven

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Figure 3. Schematic diagram of alloy solidification experiment.

Payload Design for Microgravity Science

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This detector, described in detail in ref [9], is capable of resolving from 0.1° to 1.5°C depending on the reading channel used; it is equiped with a thermoelectric Peltier cooler, that regulates the detector temperature by means of a Wheatstone bridge included in a close-loop control. The optical system based on CaFl lenses. A preamplifier averages and feeds the temperature signal to a four channel amplifier with a gain from 1 to 1000, in four steps. The output of this last device sends the preprocessed signal to an input port within the control microcomputer for digital conversion and data storage. OPERATIONAL PROCEDURE The main criteria for the infrared detector design where: power consumption, operational range, resolution, vibrational survival and size. Power consumption reached in this case 9 Watts: 60% consumed by a radiation chopper, 25% by the cooler and the rest by the amplifiers. The operational temperature range covers from 80°-1200 C with variable resolution. Launch vibrations are rather severe, thus the design is sturdy, self aligned, and fault tolerant. The payload has been submitted to the vibrational spectra obtained from a Shuttle launch, and modified according to the experience after tests. In general, modifications amounted to stiffening of several structural members, the placement of silicon dampers, and alignment verifications. The procedure to measure and control oven temperature is redundant by design. The infrared non-contact measurement is the primary system, however, the values obtained during the experiment are compared with stored values, in order to insure that the test is maintained within expected ranges at all times. In case of a significant variation, the readings from the IR detector are ignored, and control is effectuated by means of several thermocouples from which approximate temperature values are derived, so the experiment may continue despite the fault. A third control loop may also come into play in case the first and second systems fail. For such a case, temperature readings are dispensed and control is carried out on the basis of timing intervals. That is, through ground testing, melting time has been established for various environmental conditions, so as to relay the oven current input controls to the main computer programme. In this manner, the probability of total failure is reduced considerably. As a means of protection against uncontrolled power source, a set of solid state temperature sensors is provided and monitored, these allow for emergency interruption of power in the unlikely case the outside thermal insulation of the oven reaches a temperature near 35°C. The internal walls of the oven are treated with boron nitride, an effective non-wetting material that will avoid adhesion of the melted sample on to the walls, contamination, crucible stresses, and the creation of casual nucleating centers. The cooling of the sample is controlled by heater on-off sequences and by dosified nitrogen flow. The melting point for this alloy (Zn,Al-Cu;70—26—4) is 480°C, this temperature will be exceeded by about 50°C and maintained for up to 10 minutes. Controlled cooling then begins. Redundancy in the thin film deposition experiment is a result of operating three independent evaporation lobes, each provided with similar sets of crystals. Every lobe is powered by a dedicated battery set, itself diode-protected against shorts of individual units. CONCLUSIONS The integration process for a first automatic space payload has been completed, together with a test and evaluation programme. The payload has been delivered for launch, and awaits for the resumption of Shuttle flights. This type of research work, conducted within the university environment, is an important milestone for microgravity science in Mexico. At present, a second equipment is being developed, that will extend the research topics to new fields, including: multispectral remote sensing by means of a CCD cameras; production, and testing in the space environment, of autochthonous solar cells; mechanical actuators based in thermal memory alloys; electrophoresis

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of bioactive substances, and orientation sensors for future satellite integration. Despite a certain space science tradition in our country, this programme signifies a first direct involvement in production of space research hardware, and from the results and spinoffs already available, we may conclude this work will continue for many years to come. ACKNOWLEDGMENTS This work is the product of a team effort, in which the P1, besides authors, are: D. Rios, A. Oliver; and the P0: E. Vicente, J. Prado, Peralta, H. Lopez, M. Arreola. The help from Utah State U. CSE and CASS, gratefully acknowledged. The work was financed by the SCT, CONACyT UNAM.

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0. Camel, M.D. Dupouy, J.J. Favier and R. Le Maguet, Preliminary Results of the Dl—WL-GHF-04 Experiment on Dendritic Solidification of Al-Cu Alloys, XXVI COSPAR, Toulouse (1986).

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W. Kurz and D.J. Fisher, Fundamentals of Solidification, Pub. 1986.

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