- Email: [email protected]
EURECA and SGF performances
0273—1177/88 $0.00 + .50
Adv. Space Res. Vol. 8, No. 12, pp. (12)113—(12)119, 1988
Printed in Great Britain. All rights reserved.
Copyright
© 1989 COSPAR
EURECA AND SGF PERFORMANCES D. Frimout and 0. Minster European Space Research and Technology Centre, Department of Microgravity, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
ABSTRACT The EURECA platform offers unique characteristics for microgravity research: a very level micro—gravity spectrum and a long operation time for recoverable experiments.
low
Five core facilities on board will perform an impressive number of experiments. The main purpose of the Solution Growth Facility (SGF) is growing crystals at low temperatures by using the double diffusion technique in three compartment reactors. The micro—gravity eliminates the convection in the liquid, while Marangoni convection is avoided by the absence of free surfaces. The thermal gradient in the buffer zone is better than O.Ol0C/cm. Turbulence is eliminated by control of the valve rate and by a pressure compensation system. All surfaces are coated with Halar. The diffusion rate can be controlled by the use of filters. The SGF contains three independently controlled reactors. A forth reactor contains an experiment aiming at measuring the Soret coefficient of twenty binary organic mixtures and aqueous electrolyte solutions. INTRODUCTION The main advantages of the growth of crystals in low temperature solutions are on one hand the elimination of the problems related to aftergrowth cooling and on the other hand the possibility of obtaining crystals of thermally unstable materials. The growth technique employed depends strongly on the solubility of the material in the appropriate solvent and, as a rule, the problems increase as the solubility decreases. In the case of the double diffusion technique, crystal growth is achieved in a reactor made up of three chambers. The central chamber or buffer chamber is filled with pure solvent. The two reactants are dissolved each in one of the adjacent chambers. When the valves which separate the chambers are opened, the two reactants are allowed to diffuse towards the buffer chamber. The chemical reaction occurs when the two diffusion fronts meet. The concentration of the product of the reaction increases until it reaches the sursaturation levels required first for the nucleation and, further, the growth of crystals. For low solubility materials, this method is sensitive to any fluctuation such as convection or sedimentation in the solution. These fluctuations may give rise to the formation of parasitic nuclei which grow separately and one obtains finally a polycrystalline powder instead of a single crystal /1/. On earth, mass transport governed only by diffusion can be achieved by means of a gel where the solution is trapped in a flexible microporous network /2,3,4,5/. This technique avoids the sedimentation of the seeds which are usually formed by spontaneous nucleation and allows the production of crystals of improved quality. But this technique is limited to the few substances available to properly gelify the growth solutions. Moreover, the gel introduces foreign substances in the growth medium and thus impurities in the growing crystals. So, microgravity constitutes a unique environment crystals having a low solubility.
for the growth
at low temperature
of
The first attempt at growing crystals in low temperature solutions in space by means of the double diffusion technique was made by Lind in 1975 on board the Apollo Soyuz Test Project /6/.
(12) 113
(12)114
D. Frimout and 0. Minster
The experiment was carried out at ambient temperature without thermal control and the valves were operated by hand. Furthermore, bubbles had been introduced in the reactor for safety reasons. The materials which have been crystallized are calcium tartrate, calcium carbonate and lead sulfide. The results obtained are comparable to the results obtained on earth in gels regarding the number of crystals formed and their morphology. This indicated clearly that improved experimental conditions in space could lead to the growth of crystals of better quality. A second generation of space experiments has thus been carried out on board Spacelab—l with more precise experimental requirements such as — temperature control of the reactor — valves operated without generating turbulences in the liquid. — absence of gas bubbles in the liquid. The results obtained conclusively showed that the experiments under microgravity conditions are an extrapolation of gel growth methods /7,8,9/. This means that a much more wide variety of materials can be crystallized in space with an extended range of experimental conditions and growth media, as the restrictions related to the use of gels are eliminated. The following step for investigating the growth of low solubility crystals at low temperature in space has been achieved on board the LDEF (Long Duration Exposure Facility) which has been launched in 1984. TTF—TCNQ crystals were to be grown. The experiments were self contained and fully automated. The operational time was about two months. The duration of the mission was foreseen to be about 10 months. Unfortunately, no result is available yet as the LDEF is still in orbit and is scheduled to be retrieved only in 1989. Since, based on the previous results obtained in space, the experimental requirements which have been recognized for the concept of a new facility were the following ones — — — —
—
—
a thermal control is required with a precision of 0.1 OC the valve operation should avoid any perturbation of the liquid. there should be no bubble in the liquid to avoid Marangoni convection a pressure compensation system is needed to avoid the perturbations induced by the pressure differences between the chambers when the valves are operated. the inner surface of the central chamber should be as smooth as possible to avoid parasitic nucleation on the walls. the reactor should be able to contain corrosive or toxic liquids such as acetonitrile. the niicrogravity level should be below l0-~g.
This last requirement led to envisage the opportunity offered by the European Carrier for carrying out solution growth experiments in space.
Retrievable
The EUropean REtrievable CArrier : EURECA EURECA is a reusable platform which will be launched, retrieved and brought back on by the Shuttle. The total weight of EUHECA is 4000 kg of which the payload represents 1000 kg.
earth
EURECA is equipped with a propulsion system. It is thus able to reach a circular orbit at an altitude of 500 km for the duration of the mission and to come back to the Shuttle’s orbit for retrieval. Two sets of solar panels associated with Nickel—Cadmium batteries provide the 5000 N of electrical power of which 1000 W are continuously made available at the payload by means of a power bus. The duration of the first mission of EURECA will be about six months and the payload consists mainly of facilities dedicated to microgravity experiments. The experiment facilities are located on the hard points of the carbon fiber tubular structure of EURECA, or mounted on Experiment Support Panels (ESP). All the facilities are protected by a Thermal Tent under which the temperature will be kept in the range 0°Cto 30 °C. For the thermal regulation, cold plates are used which are part of the freon cooling loop of EURECA. Special constraints have been imposed for the design of the payload to achieve 5g in in theorder frequency range upa microgravity environment accelerations to 1 Hz and below l0~ g with over residual 100 Hz (see figure 1). below lO
EURECA and SGF Performances
(12)115
As EURECA will be followed by only one ground station, data transmission will be possible during five periods of about 10 minutes each per day. Therefore, the platform is equipped with a magnetic bubbles memory with a capacity of 128 Mbits. The data transmission is then achieved by means of a Packet Telemetry system which allows also in return the control of the payload from the ground. The transmission of data via the telecommunication satellite Olympus is also foreseen to be experimented as it would greatly increase the communication time available between the platform and the ground. The ESA facilities which will be installed on board EURECA in addition to the Solution Growth Facility are — the Multi Furnace Assembly : MFA — the Automatic Mirror Furnace : AMF — the Protein Crystallisation Facility : PCF — the Exobiologic and Radiation Assembly : ERA This core payload will be completed by a number of individual experiments, either microgravity relevant or not. The Solution Growth Facility : SGF The SGF is mounted on an Experiment Support Panel (ESP) of’ EURECA. Four experiments are accomodated in the SGF. Three of them are solution growth experiments using the double diffusion technique and for which the experimental requirements have been given in chapter 1. The fourth experiment aims at measuring the Soret coefficients of binary mixtures and has been designed and built by the investigator. For time optimisation purpose, all the experiments will be carried out in parallel. So, in order to be able to achieve the thermal conditions required for each experiment, four independently controlled containers are employed (see figure 2). The Power Supply and the Data Handling and Control Electronic box (DHCE, see figure 2) are common.
The containers of the crystal growth experiments ** Description
The three containers dedicated to the crystal diameter of 20 cm and a height of 77.5 cm.
growth
experiments are cylindrical
with
a
These containers have been designed to provide isothermal conditions with an accuracy of ± 0.1 °Cin the range 35 to 60°C. These isothermal conditions must be maintained during the whole duration of the mission while the surrounding temperature under the Thermal Tent of EURECA may vary between 0°Cto 30°C. The containers are made of aluminium. They are flushed with dry nitrogen prior to sealing. They are covered with a Multi Layer Isolation made up of’ 19 layers of gold—plated mylar. There is no cold plate available for active cooling. Each container is equipped with five resistance heaters which are controlled by the DHCE. Three heaters are located on the lateral wall of the container and controlled together by means of one thermistor. The two other heaters are located on the top and the bottom of the container and are independently controlled by means of one thermistor each. Due to the thermal inertia, one can expect to get an accuracy of the isothermal conditions improved by a factor 5 at the level of the crystal growth reactors. The containers are fixed vertically on an Intermediate Mounting Plate (IMP) by means of six feet. As the IMP is fixed on the ESP which temperature can vary between 0°Cand 30°C, these feet are made of titanium for thermal isolation purpose. Two containers can also be controlled by the Thermal Control System of EURECA during the dormant phases (see the mission profile). The lateral heaters controlled by one thermistor each are then employed and the temperature is regulated with an accuracy of ±10C.
(12)116
D. Frimout and 0. Minster
** Operational modes The operational modes are programmed in the microprocessor of the DHCE. The software has been conceived as flexible as possible to be able to fulfill requirements of the investigators. For the obtention of a given temperature profile, the five modes listed below available — heat up : fast heating up to a given temperature — isothermal mode : regulation at a given temperature — ramp up : heat up with a given heating rate — ramp down : cool down with a given cooling rate — oscillation mode : ramp up and down between two given temperatures
the are
The temperature profile is preprogrammed before the mission but the parameters and sequences can be modified from the ground via the Packet Telemetry system of EURECA. The new parameters can be either recorded in the mass memory of EURECA or directly transmitted to the DHCE. It should be noted that the containers can also be operated in gradient mode. This mode is employed for example in the fourth container for the measurement of the Soret coeff i— cients. The gradient is controlled by means of Peltier elements and the IMP is used as a heat sink. The thermal contact between the container and the IMP is then accurately adjusted to obtain the required thermal resistance. The crystal growth reactors The crystal growth reactors have the same design as the ones which are flown on the LDEF. As exploded view of a reactor
is shown in figure 3.
** Dimensions The reactors are cylindrical. Their dimensions have been optimized taking in account both the duration of the experiment and the rate of the process. Crystal growth will occur as long as the reactants will diffuse toward the buffer chamber and maintain the supersaturation level required for the crystallisation. Thus, the diffusion process of the reactants has been theoretically evaluated for the whole duration of the mission. Furthermore, the possibility of installing filters between the adjacent chambers and the buffer chamber exists in two reactors. The diffusion rate of the reactants toward the buffer chamber can thus be controlled to decrease the probability of’ multiple nucleation and a smaller number of crystals should be obtained but of bigger size. With a volume of the adjacent chambers of 2 liters each (external diameter 12 cm, 25 long) and a volume of the buffer chamber of 0.5 liter (external diameter 12 cm, 9 long), one can expect that the time available for the crystallisation will be used in optimum manner.
cm cm an
** Manufacturing characteristics For weight concern the reactors are made of aluminium. To be able to contain corrosive liquids and avoid the contamination of the solutions, and thus of the crystals, by metallic ions, these reactors have been internally coated. The polymer E—CTFE (ethylen and chlorotrifluorethylen), known under the commercial name HALAR, has been chosen. The HALAR presents a good resistance against a wide range of chemicals, added to suitable mechanical properties. The HALAR is also less porous than teflon and thus particularly suited for long duration experiments. This product can be easily applied on the metal and polymerized at a temperature of 4000C. In addition, the HALAR presents a smooth surface as required for the minimization of parasitic nucleation on the walls of the buffer chamber. The tightness between the chambers is achieved by means of double Kalrez 0—rings. The reactors are fixed in their container by means of two screws.
the
EURECA and SGF Performances
(12)117
** The valves The communication between each adjacent chamber and the buffer chamber is achieved by three holes with a diameter of 20 mm. The three holes of each adjacent chamber are obturated by one circular valve made of glass (fig.3). A compression spring located outside the reactor and applied on the shaft of the valve ensures the tightness of the system. The tightness of the feedthrough of each valve shaft is achieved by means of double Kalrez 0—rings. The holes are opened by a rotation of the valve of’ 600 around its axis. Each valve driven by a separate mechanism consisting of a motor and a gear box (fig.3). They can opened and closed simultaneously or separately. The valves will be rotated under microgravity conditions in 24 hours in order to the perturbation induced in the liquid. The command for rotating the controlled.
valves is sent from the
is be
minimize
ground where their position can
be
** The pressure compensation system Pressure variations will occur due to the thermal expansion particularly important in the case of the acetonitrile.
of
the liquid
which
is
A pressure compensation system is thus needed to avoid the perturbations induced by the pressure differences between the three chambers when the valves will be opened and, further, to control the pressure in the reactor which is required to stay below 1.2 atm. Therefore, a set of bellows has been placed in the three chambers of the reactor. These bellows are connected to a compensation chamber filled with argon under a pressure of 1 bar at ambient temperature. The tightness between the bellows and the chambers is achieved by means of double Kalrez 0—rings. Each adjacent chamber houses one single ring shaped bellow (fig.4). The buffer chamber contains three smaller bellows. They are integrated symmetrically in the peripheral wall to keep a spherical volume of 7 cm in diameter free for the reaction and the crystallisation (fig.5). The bellows are made of RALAR. Special attention stiffness of the bellows in each chamber.
has been paid
to obtain an
equivalent
** The filling of the reactors Special care must be taken not to trap bubbles in the reactors while filling them. Bubbles would generate Marangoni convection flows and disturb the diffusion—controlled process. One filling valve made of glass has buffer chamber must be filled through As there is no possibility of visual (fig.6). The concept of this bench is material other than glass or HALAR.
been foreseen on each adjacent chamber. Thus, the one of the adjacent chambers. inspection, a special filling bench has been built such that the solutions are never in contact with a
The filling procedure has been successfully tested no bubble in the reactors during the flight.
and one can expect that there will
be
** Diagnosis
The temperature in the three chambers of each reactor will be measured by means of the thermistors sticked on their outside surface. The pressure in the compensation systems will also be followed during the whole mission. These parameters will be measured and recorded by the DHCE once per minute. The data be transmitted each day to the ground and made available to experimenters.
will
As mentioned above there is no provision for visual observation. The accomodation of an holographic interferometer, requested by the investigators to be able to follow the whole process, was too complex and out of the scope of the available budget.
(12)118
D. Frimout and 0. Minster
** Power and electronics The Power Supply of the SGF is connected to the Power Bus of EURECA. It contains the proper electrical protections and dc—dc converters. The heaters are provided with the primary power and the valves motors and the DHCE are provided with the secondary power. When the temperature of a container is controlled by the DHCE, three separate control loops are activated : one for each of the heaters located on the top and the bottom of the container and one for the three lateral heaters. The DHCE controls the pulse—width modulation of preprogrammed operational mode.
the power to
the heaters following
the
The DHCE provides also the interface with the EURECA Data Handling System. ** Development
The Solution Growth Facility is developed (I) and Terma (DK) as subcontractors.
by Laben (I) with Contraves (CH),
Technosystem
** Mission profile A typical mission profile consist of the following phases — filling of the reactors 4 months before the launch of EURECA — launch — dormant phase during which the temperature of two containers is controlled by the Thermal Control System of EURECA. — operational phase : the containers are controlled by the Data Handling and Control Electronics of SGF. — dormant phase (up to 3 months) during which the temperature of two containers is controlled by the TCS of EURECA. — retrieval of EURECA : end of thermal control. — landing — hand over of the reactors to the investigators.
£~
cryatal growth experiments in SGF on EU~CA 1
Two of the three crystal growth experiments selected by ESA during the first mission of EURECA.
in 1985 will be carried
The first reactor will be used by Dr.K.F.NIELSEN, TU of Denmark, TTF—TCNQ crystals in acetonitrile at a temperature of 40°C.
for the
growth
out
of
The unique property of this organic charge—transfer complex is the one dimensional superconductivity like behaviour which occurs at different temperature intervals in single crystals of various perfection. Microgravity offers unique conditions allowing the obtention of crystals of better quality and bigger than on earth for the study of these particular electrical properties.
The second reactor will also be used by Dr.K.F.NIELSEN with Dr.M.D.LIND, RI, USA as co—investigator, to grow crystals of calcium carbonate in solution at 40°C. This material presents particular optical properties and is of interest on crystal growth in bio environments.
aqueous
for basic research
Both experiments also aim at better undestanding the mechanisms involved in the growth of crystals and at testing the corresponding theoretical models.
solution
The accomodation of a third newly proposed crystal growth experiment is still underway. This experiment has been proposed by Dr.M.STOECKER of the Center for Industrial Research of Norway and Drs.A.ANDERSEN and K.P.LILLERUI) of the University of Oslo and aims at synthesizing zeolite crystals in microgravity. The offretite—erionite series of zeolites is widely used as a cracking catalyser in the petro—chemical industry. Large single crystals are required for a detailed study of their structure and could not be yet obtained on earth.
EURECA and SGF Performances
(12)119
The crystals are grown using the double diffusion technique at a temperature of 90°C. Two solutions of respectively silicon and aluminium in a sodium hydroxide and potassium hydroxide based solvent are employed. Some problems are encountered for the accomodation of this experiment in the SGF as — the temperature required is out of the range available in the containers. — the solvent has a pH of 11 and attacks the parts of the reactors which are made of glass. — in case of contact between the solvent and the core of the reactor, a strong release of hydrogen is to be expected which could lead to severe hazards. The design of a special reactor made of titanium and coated with HALAR is now underway as well as the study of the extension of the temperature regulation range of the container up to 9O~C. Experiment for the measurenent of Soret coefficients in the SGF on EURECA 1 The fourth container will be used by Dr.J.C.LEGROS, ULB, the Soret coefficients of binary mixtures.
Belgium for the measurement
of
The Soret effect, or thermomigration, appears when mass transport is induced by a thermal gradient. The Soret coefficient is the ratio between the thermal diffusion coefficient and the isothermal diffusion coefficient and is defined to be positive when the denser component of the mixture migrates towards the cold regions. It has been shown that gravity—induced convection in the earth environment may not allow reliable quantitative measurement of the Soret coefficient. The experiment has been built by the investigator and will be housed in the fourth container which is of smaller sizes than the containers of the crystal growth experiments. This container will be operated in gradient mode. 20 tubes of about 1 cm in diameter and 8 cm long containing each a binary mixture will be processed simultaneously. A temperature difference of 10°Cbetween the ends of the tubes will be maintained by means of Peltier elements during the operational time of the mission. The temperature of the tubes will be continuously measured by means of one thermistor each. One of the tubes will be filled with an aqueous solution of silvernitrate and equipped with two electrodes. These electrodes will allow to follow the concentration gradient in the solution during the operational phase. At the end of the experiment, a sample of 1.5 ml will be isolated at both ends of’ the 20 tubes. The composition of these samples will be analysed after they have been brought back on the ground. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Gamier E. — These Poitiers 1985 . . .(part concerning crystal growth to be published in J. Cryst. Growth) Henish H.K., Crystal Growth in Gels (Pennsylvania University Press 1968) Arora S.K., Prog. Crystal Growth Characterization 4, (1981), 345 Wunderlich W., Cryst. Res. Technol., (1982), 17, 987 Lefaucheux F., Robert M.C., Gits S., Bernard Y., Gauthier B., Manuel Rev. mt. Hautes Temp. Refract., (1986), 23, 57—67 Lind M.D., (1977), ASTP Summary Science Report, NASA SP 412 555 Robert M. C., Lefaucheux F., Authier A. — Proceedings of the Vth European Symposium on Materials Sciences under Microgravity (1984), 193 Galster G., Nielsen K.F. — Proceedings of the 5th European Symp. on Materials Sciences under Microgravity, (1984), 189 Gerbi D.J., Egbert W.C., Ender D.A., Leung P.C.W., Rochford K.B., Virden J.W. and Cook E.L., J. Cryst. Growth 1986, 76, 673—680
Adv. Space Res. Vol. 8, No. 12, pp. (12)113—(12)119, 1988
Printed in Great Britain. All rights reserved.
Copyright
© 1989 COSPAR
EURECA AND SGF PERFORMANCES D. Frimout and 0. Minster European Space Research and Technology Centre, Department of Microgravity, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
ABSTRACT The EURECA platform offers unique characteristics for microgravity research: a very level micro—gravity spectrum and a long operation time for recoverable experiments.
low
Five core facilities on board will perform an impressive number of experiments. The main purpose of the Solution Growth Facility (SGF) is growing crystals at low temperatures by using the double diffusion technique in three compartment reactors. The micro—gravity eliminates the convection in the liquid, while Marangoni convection is avoided by the absence of free surfaces. The thermal gradient in the buffer zone is better than O.Ol0C/cm. Turbulence is eliminated by control of the valve rate and by a pressure compensation system. All surfaces are coated with Halar. The diffusion rate can be controlled by the use of filters. The SGF contains three independently controlled reactors. A forth reactor contains an experiment aiming at measuring the Soret coefficient of twenty binary organic mixtures and aqueous electrolyte solutions. INTRODUCTION The main advantages of the growth of crystals in low temperature solutions are on one hand the elimination of the problems related to aftergrowth cooling and on the other hand the possibility of obtaining crystals of thermally unstable materials. The growth technique employed depends strongly on the solubility of the material in the appropriate solvent and, as a rule, the problems increase as the solubility decreases. In the case of the double diffusion technique, crystal growth is achieved in a reactor made up of three chambers. The central chamber or buffer chamber is filled with pure solvent. The two reactants are dissolved each in one of the adjacent chambers. When the valves which separate the chambers are opened, the two reactants are allowed to diffuse towards the buffer chamber. The chemical reaction occurs when the two diffusion fronts meet. The concentration of the product of the reaction increases until it reaches the sursaturation levels required first for the nucleation and, further, the growth of crystals. For low solubility materials, this method is sensitive to any fluctuation such as convection or sedimentation in the solution. These fluctuations may give rise to the formation of parasitic nuclei which grow separately and one obtains finally a polycrystalline powder instead of a single crystal /1/. On earth, mass transport governed only by diffusion can be achieved by means of a gel where the solution is trapped in a flexible microporous network /2,3,4,5/. This technique avoids the sedimentation of the seeds which are usually formed by spontaneous nucleation and allows the production of crystals of improved quality. But this technique is limited to the few substances available to properly gelify the growth solutions. Moreover, the gel introduces foreign substances in the growth medium and thus impurities in the growing crystals. So, microgravity constitutes a unique environment crystals having a low solubility.
for the growth
at low temperature
of
The first attempt at growing crystals in low temperature solutions in space by means of the double diffusion technique was made by Lind in 1975 on board the Apollo Soyuz Test Project /6/.
(12) 113
(12)114
D. Frimout and 0. Minster
The experiment was carried out at ambient temperature without thermal control and the valves were operated by hand. Furthermore, bubbles had been introduced in the reactor for safety reasons. The materials which have been crystallized are calcium tartrate, calcium carbonate and lead sulfide. The results obtained are comparable to the results obtained on earth in gels regarding the number of crystals formed and their morphology. This indicated clearly that improved experimental conditions in space could lead to the growth of crystals of better quality. A second generation of space experiments has thus been carried out on board Spacelab—l with more precise experimental requirements such as — temperature control of the reactor — valves operated without generating turbulences in the liquid. — absence of gas bubbles in the liquid. The results obtained conclusively showed that the experiments under microgravity conditions are an extrapolation of gel growth methods /7,8,9/. This means that a much more wide variety of materials can be crystallized in space with an extended range of experimental conditions and growth media, as the restrictions related to the use of gels are eliminated. The following step for investigating the growth of low solubility crystals at low temperature in space has been achieved on board the LDEF (Long Duration Exposure Facility) which has been launched in 1984. TTF—TCNQ crystals were to be grown. The experiments were self contained and fully automated. The operational time was about two months. The duration of the mission was foreseen to be about 10 months. Unfortunately, no result is available yet as the LDEF is still in orbit and is scheduled to be retrieved only in 1989. Since, based on the previous results obtained in space, the experimental requirements which have been recognized for the concept of a new facility were the following ones — — — —
—
—
a thermal control is required with a precision of 0.1 OC the valve operation should avoid any perturbation of the liquid. there should be no bubble in the liquid to avoid Marangoni convection a pressure compensation system is needed to avoid the perturbations induced by the pressure differences between the chambers when the valves are operated. the inner surface of the central chamber should be as smooth as possible to avoid parasitic nucleation on the walls. the reactor should be able to contain corrosive or toxic liquids such as acetonitrile. the niicrogravity level should be below l0-~g.
This last requirement led to envisage the opportunity offered by the European Carrier for carrying out solution growth experiments in space.
Retrievable
The EUropean REtrievable CArrier : EURECA EURECA is a reusable platform which will be launched, retrieved and brought back on by the Shuttle. The total weight of EUHECA is 4000 kg of which the payload represents 1000 kg.
earth
EURECA is equipped with a propulsion system. It is thus able to reach a circular orbit at an altitude of 500 km for the duration of the mission and to come back to the Shuttle’s orbit for retrieval. Two sets of solar panels associated with Nickel—Cadmium batteries provide the 5000 N of electrical power of which 1000 W are continuously made available at the payload by means of a power bus. The duration of the first mission of EURECA will be about six months and the payload consists mainly of facilities dedicated to microgravity experiments. The experiment facilities are located on the hard points of the carbon fiber tubular structure of EURECA, or mounted on Experiment Support Panels (ESP). All the facilities are protected by a Thermal Tent under which the temperature will be kept in the range 0°Cto 30 °C. For the thermal regulation, cold plates are used which are part of the freon cooling loop of EURECA. Special constraints have been imposed for the design of the payload to achieve 5g in in theorder frequency range upa microgravity environment accelerations to 1 Hz and below l0~ g with over residual 100 Hz (see figure 1). below lO
EURECA and SGF Performances
(12)115
As EURECA will be followed by only one ground station, data transmission will be possible during five periods of about 10 minutes each per day. Therefore, the platform is equipped with a magnetic bubbles memory with a capacity of 128 Mbits. The data transmission is then achieved by means of a Packet Telemetry system which allows also in return the control of the payload from the ground. The transmission of data via the telecommunication satellite Olympus is also foreseen to be experimented as it would greatly increase the communication time available between the platform and the ground. The ESA facilities which will be installed on board EURECA in addition to the Solution Growth Facility are — the Multi Furnace Assembly : MFA — the Automatic Mirror Furnace : AMF — the Protein Crystallisation Facility : PCF — the Exobiologic and Radiation Assembly : ERA This core payload will be completed by a number of individual experiments, either microgravity relevant or not. The Solution Growth Facility : SGF The SGF is mounted on an Experiment Support Panel (ESP) of’ EURECA. Four experiments are accomodated in the SGF. Three of them are solution growth experiments using the double diffusion technique and for which the experimental requirements have been given in chapter 1. The fourth experiment aims at measuring the Soret coefficients of binary mixtures and has been designed and built by the investigator. For time optimisation purpose, all the experiments will be carried out in parallel. So, in order to be able to achieve the thermal conditions required for each experiment, four independently controlled containers are employed (see figure 2). The Power Supply and the Data Handling and Control Electronic box (DHCE, see figure 2) are common.
The containers of the crystal growth experiments ** Description
The three containers dedicated to the crystal diameter of 20 cm and a height of 77.5 cm.
growth
experiments are cylindrical
with
a
These containers have been designed to provide isothermal conditions with an accuracy of ± 0.1 °Cin the range 35 to 60°C. These isothermal conditions must be maintained during the whole duration of the mission while the surrounding temperature under the Thermal Tent of EURECA may vary between 0°Cto 30°C. The containers are made of aluminium. They are flushed with dry nitrogen prior to sealing. They are covered with a Multi Layer Isolation made up of’ 19 layers of gold—plated mylar. There is no cold plate available for active cooling. Each container is equipped with five resistance heaters which are controlled by the DHCE. Three heaters are located on the lateral wall of the container and controlled together by means of one thermistor. The two other heaters are located on the top and the bottom of the container and are independently controlled by means of one thermistor each. Due to the thermal inertia, one can expect to get an accuracy of the isothermal conditions improved by a factor 5 at the level of the crystal growth reactors. The containers are fixed vertically on an Intermediate Mounting Plate (IMP) by means of six feet. As the IMP is fixed on the ESP which temperature can vary between 0°Cand 30°C, these feet are made of titanium for thermal isolation purpose. Two containers can also be controlled by the Thermal Control System of EURECA during the dormant phases (see the mission profile). The lateral heaters controlled by one thermistor each are then employed and the temperature is regulated with an accuracy of ±10C.
(12)116
D. Frimout and 0. Minster
** Operational modes The operational modes are programmed in the microprocessor of the DHCE. The software has been conceived as flexible as possible to be able to fulfill requirements of the investigators. For the obtention of a given temperature profile, the five modes listed below available — heat up : fast heating up to a given temperature — isothermal mode : regulation at a given temperature — ramp up : heat up with a given heating rate — ramp down : cool down with a given cooling rate — oscillation mode : ramp up and down between two given temperatures
the are
The temperature profile is preprogrammed before the mission but the parameters and sequences can be modified from the ground via the Packet Telemetry system of EURECA. The new parameters can be either recorded in the mass memory of EURECA or directly transmitted to the DHCE. It should be noted that the containers can also be operated in gradient mode. This mode is employed for example in the fourth container for the measurement of the Soret coeff i— cients. The gradient is controlled by means of Peltier elements and the IMP is used as a heat sink. The thermal contact between the container and the IMP is then accurately adjusted to obtain the required thermal resistance. The crystal growth reactors The crystal growth reactors have the same design as the ones which are flown on the LDEF. As exploded view of a reactor
is shown in figure 3.
** Dimensions The reactors are cylindrical. Their dimensions have been optimized taking in account both the duration of the experiment and the rate of the process. Crystal growth will occur as long as the reactants will diffuse toward the buffer chamber and maintain the supersaturation level required for the crystallisation. Thus, the diffusion process of the reactants has been theoretically evaluated for the whole duration of the mission. Furthermore, the possibility of installing filters between the adjacent chambers and the buffer chamber exists in two reactors. The diffusion rate of the reactants toward the buffer chamber can thus be controlled to decrease the probability of’ multiple nucleation and a smaller number of crystals should be obtained but of bigger size. With a volume of the adjacent chambers of 2 liters each (external diameter 12 cm, 25 long) and a volume of the buffer chamber of 0.5 liter (external diameter 12 cm, 9 long), one can expect that the time available for the crystallisation will be used in optimum manner.
cm cm an
** Manufacturing characteristics For weight concern the reactors are made of aluminium. To be able to contain corrosive liquids and avoid the contamination of the solutions, and thus of the crystals, by metallic ions, these reactors have been internally coated. The polymer E—CTFE (ethylen and chlorotrifluorethylen), known under the commercial name HALAR, has been chosen. The HALAR presents a good resistance against a wide range of chemicals, added to suitable mechanical properties. The HALAR is also less porous than teflon and thus particularly suited for long duration experiments. This product can be easily applied on the metal and polymerized at a temperature of 4000C. In addition, the HALAR presents a smooth surface as required for the minimization of parasitic nucleation on the walls of the buffer chamber. The tightness between the chambers is achieved by means of double Kalrez 0—rings. The reactors are fixed in their container by means of two screws.
the
EURECA and SGF Performances
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** The valves The communication between each adjacent chamber and the buffer chamber is achieved by three holes with a diameter of 20 mm. The three holes of each adjacent chamber are obturated by one circular valve made of glass (fig.3). A compression spring located outside the reactor and applied on the shaft of the valve ensures the tightness of the system. The tightness of the feedthrough of each valve shaft is achieved by means of double Kalrez 0—rings. The holes are opened by a rotation of the valve of’ 600 around its axis. Each valve driven by a separate mechanism consisting of a motor and a gear box (fig.3). They can opened and closed simultaneously or separately. The valves will be rotated under microgravity conditions in 24 hours in order to the perturbation induced in the liquid. The command for rotating the controlled.
valves is sent from the
is be
minimize
ground where their position can
be
** The pressure compensation system Pressure variations will occur due to the thermal expansion particularly important in the case of the acetonitrile.
of
the liquid
which
is
A pressure compensation system is thus needed to avoid the perturbations induced by the pressure differences between the three chambers when the valves will be opened and, further, to control the pressure in the reactor which is required to stay below 1.2 atm. Therefore, a set of bellows has been placed in the three chambers of the reactor. These bellows are connected to a compensation chamber filled with argon under a pressure of 1 bar at ambient temperature. The tightness between the bellows and the chambers is achieved by means of double Kalrez 0—rings. Each adjacent chamber houses one single ring shaped bellow (fig.4). The buffer chamber contains three smaller bellows. They are integrated symmetrically in the peripheral wall to keep a spherical volume of 7 cm in diameter free for the reaction and the crystallisation (fig.5). The bellows are made of RALAR. Special attention stiffness of the bellows in each chamber.
has been paid
to obtain an
equivalent
** The filling of the reactors Special care must be taken not to trap bubbles in the reactors while filling them. Bubbles would generate Marangoni convection flows and disturb the diffusion—controlled process. One filling valve made of glass has buffer chamber must be filled through As there is no possibility of visual (fig.6). The concept of this bench is material other than glass or HALAR.
been foreseen on each adjacent chamber. Thus, the one of the adjacent chambers. inspection, a special filling bench has been built such that the solutions are never in contact with a
The filling procedure has been successfully tested no bubble in the reactors during the flight.
and one can expect that there will
be
** Diagnosis
The temperature in the three chambers of each reactor will be measured by means of the thermistors sticked on their outside surface. The pressure in the compensation systems will also be followed during the whole mission. These parameters will be measured and recorded by the DHCE once per minute. The data be transmitted each day to the ground and made available to experimenters.
will
As mentioned above there is no provision for visual observation. The accomodation of an holographic interferometer, requested by the investigators to be able to follow the whole process, was too complex and out of the scope of the available budget.
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D. Frimout and 0. Minster
** Power and electronics The Power Supply of the SGF is connected to the Power Bus of EURECA. It contains the proper electrical protections and dc—dc converters. The heaters are provided with the primary power and the valves motors and the DHCE are provided with the secondary power. When the temperature of a container is controlled by the DHCE, three separate control loops are activated : one for each of the heaters located on the top and the bottom of the container and one for the three lateral heaters. The DHCE controls the pulse—width modulation of preprogrammed operational mode.
the power to
the heaters following
the
The DHCE provides also the interface with the EURECA Data Handling System. ** Development
The Solution Growth Facility is developed (I) and Terma (DK) as subcontractors.
by Laben (I) with Contraves (CH),
Technosystem
** Mission profile A typical mission profile consist of the following phases — filling of the reactors 4 months before the launch of EURECA — launch — dormant phase during which the temperature of two containers is controlled by the Thermal Control System of EURECA. — operational phase : the containers are controlled by the Data Handling and Control Electronics of SGF. — dormant phase (up to 3 months) during which the temperature of two containers is controlled by the TCS of EURECA. — retrieval of EURECA : end of thermal control. — landing — hand over of the reactors to the investigators.
£~
cryatal growth experiments in SGF on EU~CA 1
Two of the three crystal growth experiments selected by ESA during the first mission of EURECA.
in 1985 will be carried
The first reactor will be used by Dr.K.F.NIELSEN, TU of Denmark, TTF—TCNQ crystals in acetonitrile at a temperature of 40°C.
for the
growth
out
of
The unique property of this organic charge—transfer complex is the one dimensional superconductivity like behaviour which occurs at different temperature intervals in single crystals of various perfection. Microgravity offers unique conditions allowing the obtention of crystals of better quality and bigger than on earth for the study of these particular electrical properties.
The second reactor will also be used by Dr.K.F.NIELSEN with Dr.M.D.LIND, RI, USA as co—investigator, to grow crystals of calcium carbonate in solution at 40°C. This material presents particular optical properties and is of interest on crystal growth in bio environments.
aqueous
for basic research
Both experiments also aim at better undestanding the mechanisms involved in the growth of crystals and at testing the corresponding theoretical models.
solution
The accomodation of a third newly proposed crystal growth experiment is still underway. This experiment has been proposed by Dr.M.STOECKER of the Center for Industrial Research of Norway and Drs.A.ANDERSEN and K.P.LILLERUI) of the University of Oslo and aims at synthesizing zeolite crystals in microgravity. The offretite—erionite series of zeolites is widely used as a cracking catalyser in the petro—chemical industry. Large single crystals are required for a detailed study of their structure and could not be yet obtained on earth.
EURECA and SGF Performances
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The crystals are grown using the double diffusion technique at a temperature of 90°C. Two solutions of respectively silicon and aluminium in a sodium hydroxide and potassium hydroxide based solvent are employed. Some problems are encountered for the accomodation of this experiment in the SGF as — the temperature required is out of the range available in the containers. — the solvent has a pH of 11 and attacks the parts of the reactors which are made of glass. — in case of contact between the solvent and the core of the reactor, a strong release of hydrogen is to be expected which could lead to severe hazards. The design of a special reactor made of titanium and coated with HALAR is now underway as well as the study of the extension of the temperature regulation range of the container up to 9O~C. Experiment for the measurenent of Soret coefficients in the SGF on EURECA 1 The fourth container will be used by Dr.J.C.LEGROS, ULB, the Soret coefficients of binary mixtures.
Belgium for the measurement
of
The Soret effect, or thermomigration, appears when mass transport is induced by a thermal gradient. The Soret coefficient is the ratio between the thermal diffusion coefficient and the isothermal diffusion coefficient and is defined to be positive when the denser component of the mixture migrates towards the cold regions. It has been shown that gravity—induced convection in the earth environment may not allow reliable quantitative measurement of the Soret coefficient. The experiment has been built by the investigator and will be housed in the fourth container which is of smaller sizes than the containers of the crystal growth experiments. This container will be operated in gradient mode. 20 tubes of about 1 cm in diameter and 8 cm long containing each a binary mixture will be processed simultaneously. A temperature difference of 10°Cbetween the ends of the tubes will be maintained by means of Peltier elements during the operational time of the mission. The temperature of the tubes will be continuously measured by means of one thermistor each. One of the tubes will be filled with an aqueous solution of silvernitrate and equipped with two electrodes. These electrodes will allow to follow the concentration gradient in the solution during the operational phase. At the end of the experiment, a sample of 1.5 ml will be isolated at both ends of’ the 20 tubes. The composition of these samples will be analysed after they have been brought back on the ground. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Gamier E. — These Poitiers 1985 . . .(part concerning crystal growth to be published in J. Cryst. Growth) Henish H.K., Crystal Growth in Gels (Pennsylvania University Press 1968) Arora S.K., Prog. Crystal Growth Characterization 4, (1981), 345 Wunderlich W., Cryst. Res. Technol., (1982), 17, 987 Lefaucheux F., Robert M.C., Gits S., Bernard Y., Gauthier B., Manuel Rev. mt. Hautes Temp. Refract., (1986), 23, 57—67 Lind M.D., (1977), ASTP Summary Science Report, NASA SP 412 555 Robert M. C., Lefaucheux F., Authier A. — Proceedings of the Vth European Symposium on Materials Sciences under Microgravity (1984), 193 Galster G., Nielsen K.F. — Proceedings of the 5th European Symp. on Materials Sciences under Microgravity, (1984), 189 Gerbi D.J., Egbert W.C., Ender D.A., Leung P.C.W., Rochford K.B., Virden J.W. and Cook E.L., J. Cryst. Growth 1986, 76, 673—680