crystal growth experiments - inaoe - p 2...crystal growth experiments on earth and in microgravity...

1

Crystal Growth Experiments on Earth and in Microgravity (μg=10-6 g)

US astronaut Marsha Ivins STS98

International Space Station (photo dated March 2014)http://images.huffingtonpost.com/2014-03-29-ISS-thumb.jpg

The aim of the School/Workshop is to prepare a selected number of participants for the next generation of projects in search of a deeper knowledge and understanding of extraterrestrial minerals and rocks, either large solid bodies or interstellar dust particles, using in-situ and remote analytical techniques.

Hanna Dabkowska, McMaster, CanadaCOSPAR-IUCr 2016

Crystal Growth Experiments on Earth and in Microgravity (μg=10-6 g)


Outline

Why and what crystals we want to grow in μg

Crystal growth on Earth

Materials feasible for growth in μg

Facilities and methods feasible in Space

Investigation of Marangoni flow: molten Si

► Apply research in development of large scale( ≥ 450mm) Si wafers production

Growth from solutions: TGS and proteins

Lessons learned and un-learned

Future directions

McMasterUniversity


● Good and large crystals are needed for many technological applications● Process of crystallization is very complex and is not well understood yet. ● Mathematical description of transport in fluids leads to difficult to solve equations* ● Models of crystal growth require details about difficult to measure materials properties. Thermophysical properties of HT melts can be measured accurately under μg● Processes observed during growth are interacting (ex. various types of convention) and tend to be turbulent. ● In microgravity conditions, as buoyancy convection is suppressed, it is possible (in principle) to have growth controlled by diffusion only. ● Diffusion becomes the predominant mechanism for transport of mass and heat.● μg helps to understand various materials phenomena in molten, fluidic and gaseous states by reducing or eliminating buoyancy - driven effects.

* Navier-Stokes equation to describe viscous flows as well as diffusion equations

Why grow crystals in microgravity ?


T1, ρ1

Buoyancy convection – high school explanationLiquid density – lower for higher temperature

T1<T2 ρ1 > ρ

2

T2, ρ2

g

Net force on the liquid volumeFB = V (ρ

1- ρ

2) g

h

Heat in

Heat out

Buoyancy (natural) convection will cease if g=0

Microgravity vs terrestrial experimentseffect of buoyancy convection


T2, ρ2

Buoyancy convection – high school explanationLiquid density – lower for higher temperature

T1 >T2 > T3 ρ

1 < ρ

2 < ρ

3

T1, ρ1

g

Net force on the liquid volume VFB = V (ρ

2- ρ

1) g

(approximation)

h

Heat in

Heat out

Buoyancy (natural) convection will cease if g=0

Surface tension force caused by temperature gradient

Microgravity vs terrestrial experimentseffect of buoyancy convection

T3


g

T2

Heat in Heat out

Marangoni thermal convection is independent on g

Surface tension of liquid:(empirical formula of Eötvös)

γ = k (Tc-T) /V2/3

K k ~ 2.1 10-7 JK-1mol-2/3

Tc critical temperature V molar volume

T1 <T2

Marangoni Convection (surface flow)

Microgravity vs terrestial experimentseffect of Marangoni convection

Marangoni convection can be caused by:temperature gradientcomposition gradient


T1

Marangoni thermal convection independent on g

T2

[Introduction to Crystal Growth and Characterization K.W.Benz, W.Neuman]

Surface tension of liquid:(empirical formula of Eötvös)

γ = k (Tc-T) /V2/3

K k ~ 2.1 10-7 JK-1mol-2/3

Tc critical temperature V molar volume

Marangoni number:

Ma = - (d γ /dT) (LΔT/η α )

γ surface tensionT temperatureL characteristic lengthΔT temperature differenceη dynamic viscosityα thermal diffusity

Thermal Marngoni convection is known as

Thermocapilary convection

Microgravity vs terestial experimentseffect of Marangoni convection


Single crystal growth is a controlled transformation from fluid phase (gas, liquid or supercritical fluid) to the solid phase*

Due to the fact that:The composition and density of fluid is not exactly the same as solidTemperature gradients are present (latent heat of crystallization !)

crystallization process requires mass and heat transfer

crystallization front diffusion

Heat flow (cooling)

stirred volume (advection)

T2 < T1

T1 < T0 = liquidus

Crystals growth in microgravity (μg) What is the difference ?

growth rate ~ supercooling

VG ~ ΔT= T0 - TG


Crystallization process requires mass and heat transfer

crystallization front diffusionlayer

Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus

latent heat released TG

Convection flows tends to turbulent and:

> fluctuation of the temperature in vicinity of crystallization front can easily exceed 10oC

►growth rate will fluctuate; can exceed critical value (crystallization front instability)

> fluctuation of fluid velocity can change thickens of diffusion layer

► effective segregation coefficient depends on growth rate and thickness off diffusion layer (composition of crystal will fluctuate)

Defects:

mosaic, grains, precipitation, striations ...

even ultra-pure silicon shows striations correlated with flow fluctuations .... (interstitial oxygen)

For “perfect” crystalsConvective flow should be eliminated/suppressed

growth rate ~ supercooling

VG ~ ΔT= T0 - TG



Crystallization process requires mass and heat transfer - convective part


Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus

latent heat released

growth rate ~ supercooling VG ~ ΔT= T0 - TG

TGMechanical stirring ~ ω2

Ex: In the Czochralski method geometry (rotating disc) forced convection effect is ~ ω2




Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG

Thermal convection ~ β g T∇

β thermal thermal expansion

g gravity acceleration

∇T temperature gradient

Mechanical stirring ~ ω2





Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG


Marangoni convection ~ - (d γ/dT) L2∇T

γ surface tension

L charact. lenght

∇T temperature gradient





Crystallization process requires mass and heat transfer


Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG




Ex: In the Czochralski method geometry (rotating disc) force convection effect is ~ ω2

Switch off rotation !!


Crystals growth in microgravity (μg) What is the difference ? What we can do?



Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG



Mechanical stirring OFF

We can not switch off thermal gradients (latent heat!)

Switch off gravity !!





Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG

Thermal convection OFF (?)



We can not switch off thermal gradients (latent heat!) or surface tension ...

Noting to switch off !!





Heat flow (cooling)


T2 < T1

T1 ~ T0 = liquidus



TG

Thermal convection OFF (?)

Marangoni convection STAYS...


We can not switch off thermal gradients (latent heat!) or surface tension on the free surface ...


Crystals growth in microgravity(μg) What is the difference ? What we can do?


● Spacecrafts provided by NASA, Russia, China and ESA

● Duration of the weightless phase (drop towers and tubes offer up to 5 seconds 7.5 inch, parabolic flights - tens of seconds, sounding rockets – minutes) very small samples, very fast growth rates, microsegregation and wetting were successfully investigated

● In space shuttle a growth can take some hours and obtained crystals can be characterized and compared with those grown on earth

● Space stations and automatic satellites allow for growth of large crystals, week(s) of experimental time and low growth rates 10K$ per pound

μg environment: facilities


● Residual accelerating vector acting on the sample – either quasi steady or fluctuating in both, amplitude

and direction. ● Stationary residual acceleration (10-6 terrestial gravity g

0) is due to:

drag the residual atmosphere on the spacecraft

the gravity gradient along the sample related to attraction exerted on it by the center of the spacecraft mass

● Fluctuating accelerations (g-jitters) are related to pumps, motors etc (10-100Hz), crew activity (0.1-10 Hz) and structure vibrations (10-2 Hz, 10-2 g)

● Solar and/or terrestrial gravity fields, related to orbital period (10-5 g

0 and 10-3 -10-4 Hz)

μg environment: residual acceleration


● Bridgman

● Floating Zone and OFZ

● Travelling Heater Method

● Drop (levitated or anchor drop)

used to synthesize “meteorites”

in lab environment

● Solution

● Vapour growth or Sublimation

http://guedel.dcb.unibe.ch/

μg environment: methods

NASA ESA Websites


● Vacuum tight metallic cartridge or fused silica ampule (8-25 x 50-300 mm, sample, crystal seed, crucible, getters, insulation, thermocouples, electrodes)

● Furnace (1200oC) with thermal gradients (multizone resistor heaters for Bridgman or VPG or mirror furnaces for OFZ)

● Electronics: to apply predetermined thermal and/or pulling programs (some real time modifications possible via remote operation) and to provide some diagnostics.

● Limitations: power consumption, size of equipment cost of delivery to the orbit

● Safety issues: high temperatures

corrosive or/and poisoning materials

μg environment: facilities and experiments


● Bridgman: USA, Russia+, Germany, Spain, France, China, Sweden

● Floating Zone and OFZ: Germany, Japan, USA● Travelling Heater Method: USA,Germany, Spain

● Drop (levitated or anchor drop): Russia,USA,Germany

● Solution: USA, Russia, India, China● Vapour growth or Sublimation: Canada, Switzerland

μg environment: countries


● Bridgman (AlSb, Bi2Ti

3, CdTe, Cd

1-xHg

xTe, InSb, GaAs, Ga

1-xln

xSb,

GaSb, Ga1-x

lnxSb, Ge, Ge

1-xSi

x, Pb

1-xSn

xTe, PbTe, Si, CeMg

3, Te

80Si

20

● Floating Zone and OFZ (BiTbSb, GaAs, InSb, GaSb, Si, Ge, InBi

● Travelling Heater Method (CdTe, Cd1-x

SexTe,InP, Ga

1-xAl

xSb,

GaAs, GaSb, Pb1-x

SnxTe,

● Drop (levitated or anchor drop) InSb, Si, also used to synthesize `meteorites`in lab enviroment

● Solution GaAs, GaP, 90% semiconductors

● Vapour growth or Sublimation (Cd1-x

HgxSe, Cd

1-xHg

xTe, Ge,

CdTe, GeSe, GeTe, HgJ2,

Pb1-x

SexTe,

μg environment: materials


● Few hundreds experiments, Salut, Mir, SpaceLab-3 (SL-3), International ml-1, etc

● 90% semiconductors

● many potential applications – experiments focused on application-oriented crystals

● industrial problems with micro and macro homogeneity were expected to subdue

● quality was expected to improve in space

● The influence of μg and vibrations on heat and mass transfer was assessed

● many characterization methods are available so it is possible to focus on solving specific problems or confirm specific theories

● Oxides and chalcogenides (Bi

12GeO

20, LiIO

3, niobates, PbBr

2, PbCl

2, RbAg

4I5)

● Protein growth from solutions

● Solidification of metalic alloys

μg environment: experiments


DyTiO and PrAlO4

congruently melted3-8 mm/h

OFZ terrestrial

26

Marangoni and buoyancy are acting in the same direction

Marangoni and buoyancy are acting in opposite directions

Z

Light intensity

Convection- Marangoni onlyVisualization only


Growth of 20 mm diameter GaAs crystals by the Optical Floating Zone Technique Duringthe D-2 Spacelab Mission [G. Miiller and F.M. Herrmann ]

GaAs Mp = 1238oCdirect gap (1.4eV) semiconductor

Microgravity FZ experiments liquid bridge stability

Liquid bridge “frozen “ in the Space – notice different shape

Terrestrial


Thermodynamical properties of HT melts: density, surface tension, viscosity, specific heat capacity and electrical conductivity, indispensable in numerical modeling,can be measured accurately in μg conditions

Without the strong buoancy convection present on Earth the effect of Marangoni convection on macrosegregation and striations in crystal growth (Si, Ge, GaAs, InSb, GaSb)was confirmed and the critical conditions were found to be in agreement with numerical predictions.

Under μg, as the weight vanishes and surface tensioncan stabilize larger zones and large crystals can be obtained(GaAs: Φ 20 mm in μg, Φ 6 mm on Earth)5 experiments,<100>,<111>, 6-8,4 mm/h, contamination with SiLess dislocationns even with 40 deg/h cooling rate Magnetic damping appliedTerrestial control without astronaut involvement

Microgravity OFZ experiments:GaAseffect of thermocapilary (Marangoni) convection

G.Miller et all, A.Croll et all, 1994


Fig. 1. (a) Representation of a floating zone of melt between two solids showing the growthangle v/ and the radio frequency heating coil. (b) Representation of a model liquid bridgebetween inert solid rods showing meridian angles Φ1 and Φ2 at the lower and upper solids.Ψ- wetting angle (≠ contact angle)[E. Boucher, M. Evans J . Chem. SOC., Faraday Trans. I , 1985, 81, 2787]

Young-Laplace equation:

P = γ (1/RX+1/RY)

P Laplace pressure γ surface tension

RX, RY radii of curvature

At any point of the surface:

P+PA+PH=0

PA ambient pressurePH = G d h hydrostatic pressure

G gravity accelerationd liquid specific massh height of liquid column

Liquid bridge can exists only within some ranges of diameter of solids, separation of solids, volume of liquid, liquid properties (density, surface tension), characteristic angles. In the case of OFZ and FZ due to the temperature gradients liquid properties are varying.

In the case of microgravity hydrostatic term is negligible and stability depends rather on oscillations (wave-like) of the bridge then of it's size

Microgravity FZ experiments liquid bridge stability

● The influence of μg and vibrations on heat and mass transfer was assessed (Russia, Ukraina)

30

Temp

Tlq

Crucible translation

furnace temperature profile

nucleationcontrolled bycrucible tip

McMasterUniversity

With natural or forced convection chemical homogenizing the liquid concentration changes continuously.

Axial segregation is perturbed by convection when liquid velocity next to the solid- liquid interface is comparable to the growth rate (usually on Earth).

More complicated radial segregation (RS) is related to the curvature of solid-liquid interface and to the level of convection. For very low convection levels liquid is homogenized by diffusion only so radial segregation is low, whereas for intermediate convection RS can be very high, with maximum when liquid velocity is in the order of growth rate

Unavoidable natural convection (buoyancy) was named the main source of problems of segregation in crystal growth as it is not possible to get diffusion only transport on Earth

Terrestrial segregation studies: Bridgman configurationChemical μ-segregation: dopants or constituents

31

about segregation....

equilibrium segregation coefficientko

-> phase diagrams

effective segregation coefficient[Burton-Prime-Slichter]

keff = ko/[ko+ (1 - ko) exp -(vδ/D)]v = growth rateδ – diffusion later thicknessD - diffusivityA A

(1-X)B

X A

(1-Y)B

Y B

mol %

Temp

T1

Miscibility in liquidMiscibility in solid (solid solution):

at temperature T1

liquid A(1-X)

BX

in equilibrium with

solid A(1-Y)

BY

Melting points of pure substances: TM,A

TM,B

TM,A

TM,Bliquidus

solidus


Bridgman crystal growth in microgravity:interaction with crucible

De-wetting: 1 μm < 60 gap between crystal and crucible (it depends on crucible material, its roughness, purity of applied atmosphere but it does not depend on the growth rate) its value agrees with the theory

CdTe crystals (from carbon coated crucibles) are better (dislocation density decreased by 100 times and there was neither grains no twins) than the best grown on Earth due to the absence of hydrostatic pressure and of the wetting phenomena.

Detached growth – reduction of crystal diameter (necking):Bubbles at the crucible-sample interface

NASA Website


Experiments on Earth and under no-buoyancy conditions proved that oscillatory Marangoni convection is responsiblefor striations in Si.

Striations decreased when the experiment was performedin μg (comparing to terrestrial conditions; Eyer et al. )

Marangoni flow is influenced by oxygen partial pressure as low as 4x10-7-2x10-5 Mpa

Those observations helped to improve the quality of Si crystals grown on Earthby FZ and by Czochralski methods

To obtain really meaningful results the diagnostic techniques must be Improved.

Striations in GaSb(Si) and InAs(Ga) are caused by convection and defects distribution.

T. Hibiya, CCGTIEM, 2003

μg FZ experimentseffect of thermocapilary (Marangoni) convection on striations

(microsegregation) in Si, GaSb(Si) and InAs(Ga)

Straitions in Y2Ti

2O

7 by OFZ

[C. Liu, BIMR McMaster, 2015]

5mm


● Sublimation (CdTe, HgJ2 in mbar pressure) is sensitive to convection as the

transport is limited by diffusion. Due to a very complicated nature of crystal growth processes no conclusive results were obtained as the convection at mbar level could not be assessed.

● CVT produced crystals bigger and better than those grown on Earth, allowing for calculation and confirming kinetic factors governing CVT reactions (Ge transported by J

2). This helped to understand chemical reactions and kinetics

● Crystals grown in sensitive to sedimentation conditions (CeMg3 or RbAg

4J

5) were

better when grown in μg conditions either due to better nucleation or because of differences in density of different phases.

● The four mass crystallization experiments (CuSO4, zeolites, brushite) were not

conclusive, but crystal quality was good

Growth from VP and other crystallizations

Rajamath, Mack Plank i


TGS is ferroelectric used for IR detectors in earth exploration, infrared detection, radiation monitoring (UV-IR) and telescopes.

● It boasts high pyroelectric coefficient and low dielectric constant

● It is monoclinic (P21

) below TC of 49oC

● Defects (gas inclusions and fluctuating dopants) limit applications and yield of crystals

● Many dopants were used in those experiments to improve crystal quality

On Earth, solution crystal growth techniques cause lowering of concentration near the growing crystal so an upward flow of solution is created. This causes a noticeable reduction of growth rate (if no stirring and T constant). For water it takes 48 min for temp change of 1 deg to be felt in 2 cm distance. To grow a crystal a forced convection and/or lowering the growth temperature is needed.

Yu. Preezant, 2003

S. Balakumar J. Mater. Chem., 2000,

Growth from Solution on Earth Triglicine Sulfate (TGS) Crystal


Preliminary experiment was carried in SpaceLab-3 (SL-3), and in International Microgravity Lab (IML-1) aboard a Space Shuttle (1985 and 1992), 18 h -7 days.Purpose was to conduct basic and applied research under μg conditions to increase understanding of fundamentals

“Cold finger technique” was used on earth for HTS growth. It allowed lowering the temperature according to pre-determined program for maintaining a supersaturated solution near the surface of the crystal.

It was possible to prove that the crystal was grown in diffusion controlled process

Dielectric losses are lower in crystals grown in IML-1 than in those grown on Earth or in SL-3

Batra et al., CCGTIEM, 2003

Triglicine Sulfate Crystal Growth in μg

Aggraval et al., CCGTIEM, 2003


To understand the role played by thousands of proteins in maintaining life one has to understand the proteins structure

In structural biology and drug design a protein crystal is used to solve its 3D structure by crystallographic methods to understand protein functions

In bio-separation; ex. insulin (Zn) from biomass

In controlled drug delivery

http://chemistry.elmhurst.edu/

Crystallization of Proteins


● μg provides convection and sedimentation-free environment

● Some proteins grow better and bigger than on Earth but not all of them. NASA funded many thousands individual protein crystal growth experiments in μG, new theoretical models were presented

● Multiuser Protein Crystallization Apparatus for Microgravity (PCAM) was developed, neutron size crystals were grown

● Diffusion-Controlled Crystallizator (DCAM) utilizes dialysis and allows to passively control each experiment for few days to several months. Developed at Marshall Space Flight Center to grow protein crystals by a special diffusion process.

● On ISS the x-ray analysis is being done on-board

● DCAM on Mir STS-89 in January 1998 ● developed at Marshall Space Flight Center to grow protein crystals by a special diffusion process.

Proteins grown in μG conditions


A strict protocol is needed to reproducibly produce x-ray quality protein crystals from supersaturated solution by precipitation or crystallization in closed systems (hanging drop method or sitting drop method)

Solvent evaporation, change of ionic strength, pH, concentration, purity, temperature

Optimization of the sample preparation, crystallization conditions and device to enhance micro-gravity effect.

To launch crystallization of protein samples without anyoptimization would likely lead to inconclusive results

Crystal growth mechanism can be observed

and explained by atomic force microscopy

Protein Crystal Growth in μg

insuline


[ From: http://www.hngn.com/articles/189693/20160317/scientists-grow-crystals-on-the-international-space-station-with-new-experiments.htm 2016-03-17 ]

Crystals grow in space on the International Space Station. (Photo : K. Tsukamoto et al/Tohoku University)

Very slow growth and dissolution rate of protein crystals was monitored .

"We are interested in the growth mechanisms of space-grown lysozyme crystal as a model to understand why space-grown crystals sometimes do show better quality than the Earth-grown crystals," Tomoya Yamazaki, The experiment was performed on the ISS in 2012. The researchers took precise measurements of the growth rate of the lysozyme crystals versus the driving force, supersaturation. They also looked at measurements of the solution's refractive index distribution.

The space experiment NanoStep, in the Japanese Experimental Module (KIBO) of the ISS in 2012. in Solution Crystallization Observation Facility (SCOF) onboard KIBO.The surfaces of the protein crystals and refractive index of solution were observed and their growth rates were measured by a Michelson-type Mach–Zehnder-type interferometry.

Time of experiment: months

μg environment: facilities and experiments

http://www.hngn.com/articles/189693/20160317/scientists-grow-crystals-on-the-international-space-station-with-new-experiments.htm


Human Antithrombin III controls blood coagulation in human plasma. Its importance is underscored by the occurrence of severe thrombotic disorders including deep vein thrombosis, pulmonary embolism, and cerebral infarction in subjects with antithrombin mutations. Antithrombin is commonly given to patients suffering thromboticcrises of the shock syndromes. Investigator: Dr. Mark R. Wardell, Washington University School of Medicine, St. Louis, Mo.Lysozyme is used a protein model to document the effects of microgravity on crystal growth.Investigators: Dr. Daniel C. Carter, New Century Pharmaceuticals, Huntsville, Ala., Dr. Franz Rosenberger and Dr. Bill Thomas, University of Alabama in Huntsville.Nucleosome core particles have important roles in the regulation of gene expression, particularly in theexpression of genes transcribed by RNA polymerase III. The nucleosome is the basis for organization within the genome by compacting DNA within the nucleus of the cell and by making selected regions of chromosomes available for transcription and replication. Investigator: Dr. Gerard J. Bunick, Oak Ridge National Laboratory, Oak Ridge, Tenn.Outer surface glycoprotein of the hyperthermophile Methanothermus fervidus lets M. fervidus live under environmental extremes, like high temperature, low-pH value, or high salt concentration. Elucidation of the crystal structure of this glycoprotein,which is directly exposed to the environment, may provide important information on the survival of these unusual microorganisms. Investigator: Dr. Jean-Paul Declercq, UniversitĂŠ Catholique de Louvain, Belgium.Serum Albumin, a key ingredient in blood plasma, is crucial to the transport of drugs and other chemicals throughout the body.Investigator: Dr. Daniel Carter, New Century Pharmaceuticals, Huntsville, Ala.HK-Gro EL Complex is used in fundamental virus structure and function studies. Investigator: Dr. John Rosenberg University of Pittsburgh.Ferritin and Apoferritin are used in fundamental biochemistry and crystal growth model systems.Investigators: Dr. Franz Rosenberger, Dr. Bill Thomas, University of Alabama in Huntsville, Dr. Daniel C. Carter, New Century Pharmaceuticals, Huntsville, Ala.Bacteriorhodopsin has potential for storing data in optical computer crystals. Investigator: Dr. Gottfried Wagner Justus-Liebig-University, GermanyFerrochelatase is important to biomedical and biochemical applications. Investigators: Dr. B.C. Wang and Dr. Harry Dailey University of Georgia.

Proteins grown in μG conditions


The leading role of thermocapilary (Marangoni) convection (not buoyancy) on striations in FZ crystals is confirmed (even for terrestrial processes)

Dewetting processes (Bridgman) helped to understand generation of twins, grains and dislocations as CdTe crystals grown in Space have considerably less defects than those grown on Earth

Not all processes are fully explained, but the effect of natural convection on axial chemical segregation in crystals of diluted alloys is solved (Bridgmen)

● Vapour Growth helped to understand chemical reactions and kinetics

Benefits of CG experiments in μg


● New generations of space facilities have to be build, with in-situ diagnostic capabilities, to gather and assess in real time experimental data enabling to process materials at higher temperatures and in μg

● This will allow mankind to take advantage of gravity related physical phenomena affecting crystal growth processes

Future directions - after 35 years


How Do We Know It's Really Microgravity?

When you see astronauts floating around the International Space Station and hear they are in microgravity, what does that really mean? We know that the astronauts appear weightless, but the space station and all of its contents, including the astronauts, actually still are under the influence of Earth's gravitational pull. This is because the station remains in orbit around our planet. Both station and astronauts are actually falling around Earth.

A great way to visualize how orbits work is to play this Shoot a Cannonball into Orbit game, demonstrating the physics of acceleration and gravitational force. Both spacecraft and crew are going at the correct speed horizontally to maintain their altitude above Earth, and together experience a continual state of free fall. This near weightlessness is commonly referred to as "microgravity." Gravity on Earth is abbreviated as 1g. The prefix "micro-" refers to one-millionth, so that microgravity implies 1/1,000,000 of Earth's gravity.

For various reasons, this ratio is not constant; however, the deviation is slight enough that, for simplicity's sake, we describe it as a constant. For example, at the space station's current altitude, about 240 miles above us, there still is a very thin atmosphere that imparts aerodynamic drag, and the drag changes slowly as the space station orbits Earth. This drag counteracts the station's free fall to a very small degree; but, it is still there and plays a part in scientific investigations.

Microgravity researchers take advantage of the fact that their experiments, along with everything else in the space station, are free falling. While in orbit on the space station, these investigations do not experience the effects of gravity, such as buoyancy, convection or sedimentation. These effects, which tend to cause fluids to move and mix in our 1g environment here on Earth, are greatly reduced in orbit.

The precise value and fluctuations of the microgravity environment are important in interpreting the data from station investigations. There are accelerometer systems in orbit to measure the microgravity environment. Two of those systems are sponsored by NASA's Glenn Research Center in Cleveland: the Space Acceleration Measurement System (SAMS) and the Microgravity Acceleration Measurement System (MAMS).


AMS measures low frequency, low magnitude vibrations or accelerations below 0.01 hertz. This is one vibration every 100 seconds, which is very slow. Typically, large massive structures vibrate slowly. These accelerations include the effects of aerodynamic drag that are typically smaller than one micro-g. The nature of these accelerations is such that measurements can be made at one location and applied to any other location on the space station.

SAMS, on the other hand, deploys multiple accelerometers throughout the space station to measure higher frequency accelerations, between 0.01 to 300 hertz, which typically range from 10 to several thousand micro-g. These vibrations require measurements close to the point of origin and tend to come from equipment like fans and pumps or from crew activity such as exercise. In addition, SAMS measures transient vibrations, which are relatively brief and fairly strong. These accelerations can be caused by crew movement pushing off bulkheads and landing, from vehicle thrusters to maintain attitude or reboost altitude, vehicle dockings and machinery start up. Such activities can produce peak measurements of more than ten-thousand micro-g's.

"SAMS also plays an important role in monitoring the space station's structural integrity," said Ken Hrovat at Glenn. "SAMS' measurements are analyzed to determine the exact nature of flexing and bending of important space station structures, its 'backbone,' to assess vehicle longevity." Recent analysis suggests that the space station will be sturdy and safe enough as a microgravity research platform until about the year 2028.

As a result of the measurements collected by SAMS and MAMS, researchers are able to monitor continuously the true nature of and small changes in the microgravity environment on the space station. Researchers at Glenn receive the data from these instruments as it streams down from station, displaying in near real-time on the Web. An archive of the data provides this information to sustaining engineering, scientific investigators and the microgravity community at-large.

'How do we know it's really microgravity?' Thanks to SAMS and MAMS we sense its effects and we measure it! Last Updated: Feb. 4, 2016Editor: NASA Administrator


Nav

ier-

Sto

kes

Equ

atio

ns

Re = ρvL / μ Inertial forces/viscous forces

Pr = ν / αViscous diffusion/thermal diffusion

Density ρ Velocity v Characteristic length LDynamic viscosity μ Cinematic viscosity νThermal diffusivity α

crystal growth experiments - inaoe - p 2...crystal growth experiments on earth and in microgravity...

Documents