1. Crystallization Techniques and Materials for Double Beta Decay Studies
I.Dafinei
INFN Sezione di Roma, ITALY
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2. content introduction crystal growth
nucleation and growth
crystal growth methods
crystal growth from the melt
crystals for Double Beta Decay (DBD)
DBD application constraints
TeO2 “case study”
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3. introduction (1) low temperature detectors (LTD)
CUORICINO/CUORE
4 x 760 g detectors
very good energy resolution (<0.01eV)
very low energy threshold
sensitivity to non-ionizing events
practically unlimited choice for energy absorber
material (radio)purity is the major limitation of LTD use in Rare Events Physics applications
material synthesis
detector construction
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4. introduction (2) why crystal growth ? research applications
properties of solids may be obscured by grain boundaries
metals
insulators
semiconductors
superconductors
protein crystals …
uniform properties on microscopic level
electronics
optics
mechanics
micro-devices
~1 wafer/mm
~ 106 devices
1 boule
very high stability in time
beautiful
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5. nucleation and growth present: bismuth garnet quartz
W. Kossel, Nachr.Ges.Wiss.Gottingen, Math.physik Klasse, 1935 (1927), Ann.Physik., 21, 455 (1934) ; 33, 651 (1338)
Kossel: growth from vapour and from solution
models:
Lennard-Jones: growth from a melt
0.0662 2
0.1807 1
0.8738
relative probability
position
in the case of NaCl
quartz
present:
Monte Carlo modeling of reactions on the surface of the growing crystal
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6. crystal growth methods
generally classified as:
melt growth
solution growth
vapor growth
each method has several different versions
directional solidification from the melt ~ mm/hr
supersaturation ~ mm/day
sublimation-condensation ~ µm/hr
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7. growth from the melt (1) feasibility conditions: congruent melting
not trivial in the case of binary or more compounds
existence region
congruent melting x T1 T2 T3 T4
incongruent melting
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8. growth from the melt (2) feasibility conditions (continued):
raw material must not decompose before melting
changes in stoechiometry of the melt due to different evaporation rates are also to be avoided
grown crystal must not undergo a solid state phase transformation when cooled down to room temperature
preliminary detailed study of phase diagram is needed
thermodifferential analysis
thermogravimetrical analysis
X-ray diffraction analysis …
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9. growth from the melt (3) example PbO-WO3 compounds WO3 PbO 1123° 970°
Chang, 1971
1100
1000
900
800
700 20 40 60 80
mole %
WO3
PbO
Liquid
16.5%
37%
66.5%
1123°
970°
740°
915°
935°
730°
2:1
1:1
PWO4
PWO3
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Phase diagram of PbO-WO3 binary compounds
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10. growth from the melt (4) characteristics
fast (~mm/hr) growth rate is limited by heat transfer, not by mass transfer
allows for a large variety of techniques
Verneuil
Bridgman-Stockbarger
Czochralski-Kyropoulos
zone melting and floating zone
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11. Verneuil vibration growth characteristics: 1902, Auguste Verneuil
no crucible contamination
highly pure starting material (> %)
strict control of flame temperature
precise positioning of melted region
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12. Bridgman-Stockbarger (1)
temperature
Tmelt
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characteristics:
charge and seed are placed into the crucible
no material is added or removed (conservative process)
axial temperature gradient along the crucible
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13. Bridgman-Stockbarger (2)
the shape of the crystal is defined by the container
no radial temperature gradients are needed to control the crystal shape.
low thermal stresses result in low level of stress-induced dislocations.
crystals may be grown in sealed ampules (easy control of stoichiometry)
relatively low level of natural convection
easy control and maintenance
advantages
confined growth (crucible may induce stresses during cooling)
difficult to observe seeding and growing processes
changes in natural convection as the melt is depleted
delicate crucible and seed preparation, sealing, etc.
drawbacks
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14. Bridgman-Stockbarger (3)
melts with volatile constituents:
III-V compounds (GaAs, lnP, GaSb)
II-VI compounds (CdTe)
ternary compounds:
Ga1-xlnxAs, Ga1-xlnxSb, Hg1-xCdxTe
applications
improvement example (liquid encapsulation)
crucible
encapsulant
melt
crystal
reduced nucleation
reduced thermal stresses
reduced evaporation
prevents contact between crucible and melt
B2O3
LiCl, KCl, CaCl2, NaCl
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Best encapsulans:
- B2O3
- LiCl, KCl, CaCl2, NaCl
low vapor pressure
melting temperature lower than the crystal
density lower than the density of the melt
no reaction with the melt or crucible
encapsulant characteristics
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15. Czochralski-Kyropoulos (1)
Jan Czochralski ( )
seed
grown crystal
molten raw material
heating elements
Kyropoulos
Czochralski
1918
1926
A seed crystal mounted on a rod is dipped into the molten material. The seed crystal's rod is pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, a single-crystal cylindrical ingot is extracted from the melt. The process may be peformed in controlled atmosphere and in inert chamber.
characteristics:
charge and seed are separated at start
no material is added or removed (conservative process)
charge is held at temperature slightly above melting point
crystal grows as atoms from the melt adhere to the seed
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16. Czochralski-Kyropoulos (2)
growth from free surface (stress free)
crystal can be observed during the growth process
forced convection easy to impose
large crystals can be obtained
high crystalline perfection can be achieved
good radial homogeneity
advantages
delicate start (seeding, necking) and sophisticated further control
delicate mechanics (the crystal has to be rotated; rotation of the crucible is desirable)
cannot grow materials with high vapor pressure
batch process (axial segregation, limited productivity)
drawbacks
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advantages
growth from free surface (stress free)
crystal can be observed during the growth process
forced convection easy to impose
large crystals can be obtained
high crystalline perfection can be achieved
good radial homogeneity
Drawbacks
delicate start (seeding, necking) and sophisticated further control
delicate mechanics (the crystal has to be rotated; rotation of the crucible is desirable)
cannot grow materials with high vapor pressure
batch process (axial segregation, limited productivity)
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17. zone melting (1) characteristics:
ultra-pure silicon
characteristics:
only a small part of the charge is molten
material is added to molten region (nonconservative process)
molten zone is advanced by moving the charge or the gradient
axial temperature gradient is imposed along the crucible
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18. zone melting (2) advantages drawbacks advantages Drawbacks
Charge is purified by repeated passage of the zone (zone refining).
Crystals may be grown in sealed ampules or without containers (floating zone).
Steady-state growth possible.
Zone leveling is possible; can lead to superior axial homogeneity.
Process requires little attention (maintenance).
Simple: no need to control the shape of the crystal.
Radial temperature gradients are high.
advantages
Confined growth (except in floating zone).
Hard to observe the seeding process and the growing crystal.
Forced convection is hard to impose (except in floating zone).
In floating zone, materials with high vapor pressure can not be grown.
drawbacks
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advantages
Charge is purified by repeated passage of the zone (zone refining).
Crystals may be grown in sealed ampules or without containers (floating zone).
Steady-state growth possible.
Zone leveling is possible; can lead to superior axial homogeneity.
Process requires little attention (maintenance).
Simple: no need to control the shape of the crystal.
Radial temperature gradients are high.
Drawbacks
Confined growth (except in floating zone).
Hard to observe the seeding process and the growing crystal.
Forced convection is hard to impose (except in floating zone).
In floating zone, materials with high vapor pressure can not be grown.
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19. high purity solvent insoluble in the crystal
other methods (1)
growth from solutions
melt non congruently
decompose before melting
have very high melting point
undergo solid state phase transformation between melting point and room temperature
key requirement
high purity solvent insoluble in the crystal
molten salt (flux) growth
a liquid reaction medium that dissolves the reactants and products, but do not participate in the reaction
flux:
oxides with very high melting points
PbO, PbF2, B2O3, KF
very slow, borderline purity, platinum crucibles, stoichiometry hard to control
carried on at much lower temperature than melting point
typical solvents:
main advantage:
limitations:
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20. other methods (2) liquid phase epitaxy hydrothermal growth advantage
lower temperatures than melt growth
high quality layers of III-V compounds (Ga1-xlnxAs, GaAsxP1-x)
GaAs and GaSb from Ga solution
limitation
very slow, small crystals or thin layers
aqueous solution at high temperature and pressure
typical example: quartz industry
SiO2 is grown by hydrothermal growth at 2000 bars and 400°C because of α-β quartz transition at 583°C
hydrothermal growth
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21. crystal purity (1)
Solubility of possible impurity is different in crystal than melt, the ratio between respective concentrations is defined as segregation coefficient (k0)
impurity equilibrium concentration in crystal
impurity equilibrium concentration in melt
As the crystal is pulled impurity concentration will change in the melt (becomes larger if segregation coefficient is <1). Impurity concentration in crystal after solidifying a weight fraction M/M0 is:
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22. crystal purity (2) The effective segregation coefficient (ke):
As a consequence, floating zone method will give crystals with lower concentration of impurities having k<1 than Czochralski growth
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multiple pass may be run in order to achieve the required impurity concentration
there is no contamination from crucible
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23. ββ emitters of experimental interest
crystals for DBD
DBD application constraints
impurity allowed (g/g):
T = 1018 – 1024 yr
usual Ti < 1012 yr
close to detection limit of the most sensitive techniques used for quantitative elemental analysis (NAA, ICP-MS)
ββ emitters of experimental interest
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24. TeO2 crystal (1) TeO2 (paratellurite) relatively low melting point
short:: 1.88 Å
long:: 2.12 Å
relatively low melting point
distorted rutile (TiO2) structure
anisotropy of expansion coefficient
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Tellurium dioxide can be found in nature in two forms: tellurite (orthorombic) and paratellurite (tetragonal). Paratellurite is by far more interesting for its acoustic and optical properties.
Paratellurite (α-TeO2) has a distorted rutile (TiO2) structure with asymmetric covalent Te-O bonds. The short bonds (1.88 Å) are indicated by dashed green lines and long bonds (2.12 Å) by full violet lines. One of the C2 symmetry axis is shown.
Crystals are colorless and highly transparent in the range of 350 nm - 5 μm. The density of grown crystals is 6.04 g/cm3, measured lattice constants: a = Å and c = Å.
Crystals are grown from melt (melting point at 733 °C) Note the relatively low melting point which in principle should make the crystal growth not very complicated which is not true in TeO2 case (melt hydrodynamic instability, high anisotropy of expansion coefficient).
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25. raw material preparation
TeO2 crystal (2)
raw material preparation
TeO2+HCl→TeCl4+H2O
TeO2
2Te+9HNO3 → Te2O3(OH)NO3+8NO2+4H2O
Te2O3(OH)NO3→2 TeO2+HNO3
TeCl4+4NH4OH→Te(OH)4+4NH4Cl
Te(OH)4→TeO2+H2O
HNO3
HCl
TeCl4
NH4OH Te
washing
filtering
drying
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Powdered tellurium dioxide used as raw material for crystal growth is typically obtained by “wet” methods consisting of successive chemical reactions, washings, filterings and dryings. Nitric and hydrochloric acids and ammonium hydroxide are used in this process which gives at the end powders of typical % purity.
Crystals may be grown by Czochrlaski or Bridgman in Pt crucibles (short comment on each method peculiarities).
In principle Bridgman grown crystals should be more stressed than Czochralski ones but annealing at about 550°C helps in removing the residual stresses.
==================================
HNO3 nitric acid
NO2 nitrogen dioxide
HCl hydrochloric acid
NH4OH ammonia (Ammonium Hydroxide)
NH4Cl ammonium chloride
Te(OH)4 tellurium hydroxide
Te2O3(OH)NO3 tellurium nitrate
TeCl4 tellurium chloride
TeO2 tellurium dioxide
=======================================
TeO %
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26. TeO2 crystal (3) crystal growth
seed
grown Xtal
molten TeO2
heating
Czochralski
Bridgman
Bridgman grown crystals are more stressed than Czochralski ones
annealing at about 550°C helps in removing the residual stresses
TeO2 crystal is particularly repellent to impurities
most of radioactive isotopes have ionic characteristics incompatible with substitutional incorporation in TeO2
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27. TeO2 crystal (4) Te possible substitutional ions in TeO2
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238U (T=4.5·109 yr)
184W (T=3·1017 yr)
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28. TeO2 crystal (5) radiopurity natural radioactivity activation products
crucible material
main radioactive series
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Besides the natural repellence of foreign ions in TeO2 lattice mentioned before, there are other facts which contribute to a certain optimism concerning the radiopurity of grown TeO2 crystals.
If we consider as radioactive contamination risk those elements:
belonging to main radioactive series
having natural radioactive isotopes
having ionic radius close to Te4+ and neutron activation radioactive isotopes
A peculiar attention has to be devoted to Pt because as it stays in direct contact with the melt during the growth process
Note that most of radioactive isotopes have ionic characteristics incompatible with substitutional incorporation in TeO2 crystal lattice. It is expected therefore a larger than usual purification effect through crystal growth
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29. conclusion shares of 20 000 tons, world crystals production in 1999
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ECAL-CMS: (80 tons PWO)/
CUORE: (1 ton TeO2)/?
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