Coriell, S. R.

Duval, Walter M. B.

Singh, N. B.

Su, Ching-Hua

NASA Technical Reports Server

Double-diffusive convection during growth of halides 
This presentation is a science requirement document for a flight experiment
N B Singh and S. R. Coriell
University of Maryland, Baltimore County, Baltimore MD 21250, USA
Ching-Hua Su
Marshall Space Flight Center, Huntsville, Alabama, 35812, USA
Walter M B. Duval
NASA Glenn Research Center, Cleveland, OH 44135, USA
Email: singna@umbc.edu
Phone: (410) 455-3427
• Acknowledgement: 
• Dr. R. Hopkins, Dr. Robert Maelsky, Westinghouse R&D
https://ntrs.nasa.gov/search.jsp?R=20150021447 2019-08-31T05:47:43+00:00Z
Outlines
• Background on Materials Systems
• Heavy metal halides and selenides
• PbBr2
• Hg2Cl2
• Science Requirement Document (SRD): Need for microgravity experiment
• Bridgman Growth
• Morphological developments during melt growth
• Toroidal Instabilities
• Line defects and Point defects
• Performance of material
• Physical Vapor Transport Growth
• Preparation of microgravity experiment
• Flight Experiment
• Characterization of crystals
• Summary 
We will present examples of Bridgman and PVT growth  experiments
Double-diffusive convection during growth of halides
AOTF Crystals: leader in producing Efficient EO 
crystals
Acoustic Wave Velocity Transmission
AO Crystal X 10-5 cm/sec Range (um) Figure of Merit 
Relative to Si “M2”
Fused Silica 5.96 0.20 - 4.5 1
PbMoO4 3.63 0.42 - 5.5 23.9
TeO2 0.620 0.35 - 5.0 695
PbCl2 2.51 0.30 - 20 135
PbBr2 2.30 0.35 - 30 550
Hg2Cl2 0.347 0.35 - 20 700
Hg2Br2 0.273 0.40 - 30 2600
Hg2l2 0.254 0.45 - 40 3200
Tl3AsSe3 1.05 1.2 - 16 2800
AgTlSe 1.02 0.80 - 20 1000
TIPSe4 2.00 0.80 - 9.0 1370
Tl3AsS4 2.15 0.60 - 12 416
Examples of  the Highest FOM Crystals Which Cover a Very Large 
Transparency Range
These crystals are key for the hyperspectral 
imagers for variety of applications
Target
Red
Blue
Green
To identify 
the bad guy
AOTFs Can Be Optimized for specific Mission: Face Recognition, 
Airborne Detection Of Ground Vehicles, Airborne Detection of Trace 
Gases for UGF Detection & Localization
Wavelength filtering AOTF covering 
visible to LWIR 
Range
Crystal
Detector Array
These crystals are suitable for Low cost, large volume 
multifunctional detectors (g-ray and MWIR and LWIR) 
• Low cost source materials (Avoid rare earth elements)
• Low temperature process
• Large volume production
• Avoid materials which require special handling 
– Hygroscopic
– Fabrication/special electrode bonding etc
Cost : Success of NaI is good example
Performance: CZT is good example
Cost, quality and timing and multifuctionality approach
(Heavy metal halides-Chalcogenides)
We have grown and fabricated devices from 
large and small bandgap selenides and halides  
 Selenides (Pure and Doped)
 GaSe, TGaSe2
 PbSe
 Tl2Se
 HgSe
 ZnSe, ZnS
 Tl3AsSe3, Tl3AsSe3-xSx,  
TlGaSe2, AgGaGe3Se8, 
AgGaGe5Se12
 Halides
 Hg2Cl2, Hg2Br2, Hg2I2 Hg2Br2-xIx
 PbCl2, PbBr2 and PbI2
 TlBr, TlI and TlBrxI1-x
 Tl4HgI6, TlPbI3 Tl3PbI5 ,Tl3PbBr5, 
Tl3HgI5, Tl3HgI5
Modeling of convection for the growth is essential 
when we scale the size 
Growth of large crystal is hampered by convection during growth
Outlines
• Background on Materials Systems
• Heavy metal halides and selenides
• PbBr2
• Hg2Cl2
• Science Requirement Document (SRD): Effect of convection and 
need for microgravity experiment
• Bridgman Growth
• Morphological developments during melt growth
• Toroidal Instabilities
• Line defects
• Point defects
• Physical Vapor Transport Growth
• Preparation of microgravity experiment
• Flight Experiment
• Characterization of crystals
• Summary 
We will present examples of Bridgman and PVT growth  experiments
Double-diffusive convection during growth of halides
For the direct observation we chose a transparent 
material system: PbBr2
Cracking takes place due to phase transition in pure PbBr2
Lead bromide goes through a 
phase destructive phase 
transition (Tetragonal to 
orthorhombic)
Growth of PbBr2 and PbI2 crystals was performed 
in transparent furnaces to control the interfaces
Massive cracking takes place during scale up
Modeling was performed to evaluate growth velocity for 
different thermal gradients for planar interfaces
Massive cracking takes place due to phase transition
Morphologcal stability limit was 
determined for Solidification 
velocity and concentration  of 
dopant
Morphological stability and convective stability 
were studied for different growth velocities
Higher concentration doping was possible for low -g
Growth of crystal was performed in transparent 
furnaces to control the interfaces
Growth of crystal in transparent furnace
Temperature gradient, growth speed and dopant 
concentration controls the shape of  solid-liquid 
interface (20K/cm)
As the size of crystal (diameter) increases, convective forces dominate
Growth interface and instabilities in lead halides
Sequential development 
toroidal of instability
Instability is root cause of line and point defects
Example of S/L breakdown for 5000ppm doped lead halide; 
Double diffusive convection plays very important role
Interface instabilities in large diameter ampoules 
turned into dendritic patterns
Sequential developed of instabilities, 
interface breakdown and defect generation
17
Role of double diffusive convection was 
experimentally demonstrated
Torroidal Instability 
Experimentally Demonstrated
The convective instability can be understood by defining solutal and thermal Rayleigh numbers based on the diffusion length 
DL/V, namely, 
Rs   = BCBg(DL/V)3     , 
DL
Ra   = gGLg(DL/V)4
L (3)
where ß is the solutal expansion coefficient, CB is the silver bromide concentration, DL is the diffusion coefficient in the melt,  is 
the kinematic viscosity, g is the thermal expansion coefficient, and  is the thermal diffusivity of the melt. 
A large line defect and void in the crystal results due 
to instability. Diameter of the ampoule is 25mm
Growth of crystal in transparent furnace showing line defects and 
mechanism for torodial instability
L
S
S
Interface shape during growth
1-g grown Crystal and Etchpit density
Etchpit density showed effect of stresses due to convection
Effect of Thermosolutal effect on the 
quality of crystal
Quality was function of Rayleigh number indicating 
effect of convecto-diffusive growth 
Some parts of Lead halide –selenide
crystals show good quality
X-ray rocking curve and 2 theta-omega curve for a crystal.
Bulk transparency 
of crystal with and 
without wire mesh 
shows good quality
Crystals showed good quality
Etchpit density for doped crystals
Etchpit density increased as the convection level  (Rs) increased
A typical electrical measured resistivity and dielectric 
constant of halides
__________________________________________________________
Parameters PbI2 PbBr2 PbI2-xSex
__________________________________________________________
Resistivity (-cm) 6x 109 5x 107 8x1010
Dielectric Constant 11.8 30.7 9
___________________________________________________________
Mixed halides had highest resistivity and lowest dielectric constant
Resistivities are several orders of magnitude lower than 
HgI2.  But these are very preliminary materials.  We are 
optimzing the materials and processes
Mixed halides showed good 
fabricability
Detector of pure salt showing charge 
collection up to 60 seconds
Charge collection up to 60 seconds
Double-diffusive convection during growth of halides
Outlines
• Background on Materials Systems
• Heavy metal halides and selenides
• PbBr2
• Hg2Cl2
• Science Requirement Document (SRD): Need for microgravity experiment
• Bridgman Growth
• Morphological developments during melt growth
• Toroidal Instabilities
• Line defects and Point defects
• Performance of material
• Physical Vapor Transport Growth
• Preparation of microgravity experiment
• Flight Experiment
• Characterization of crystals
• Summary 
We will present examples of Bridgman and PVT growth  experiments
Impurities act as solute and cause solutal convection
High purity section of crystal showed lowest scattering
Thermosolutal convection cause inhomogeneity
PVT growth:  Oriented seeds were used to avoid 
anisotropic contribution
We used two zone transparent furnaces
Hg2Cl2 Crystals with large Natural facets
)(
8
4
PePs
l
a




)(
8
224
TcTs
PcPs
lR
a
N





The velocity profile across the circular duct is derived by substituting 
the general expression for shear stress into Newton’s law of viscosity and 
integrating by applying the no-slip condition at the wall. The average 
velocity is defined as:
 = Volume Flow Rate
 = Viscosity
Number of Moles per second 
through cross section
where P = PS – Pc,  PS and Pc are the vapor pressure at the source 
and crystal interfaces,  is the viscosity, 8 L a result  which is 
Hagen-Poiseuille equation.  In spite of the fact that this equation is 
only applicable to incompressible fluids, one can derive the number 
of moles, N, flowing each second through the cross sectional area, 
which is given as:
2a
Ts Ps
Tc Pc
l
PVT in Vertical Geometry
Velocity Equation
)(
6.9 222
TcTs
PcPs
l
Ma
V
s 



a and l in cm
M in g/mole
 in Poise
s in g/cm
3
Pc and Ps in Torr
Tc and Ts in K
A  growth  rate higher than 5 cm/day was predicted. 
PVT growth and velocity equation
Hg2Cl2 Crystals with favorable properties for 
detectors
Heavy metal halides were use for 0.3 to 30mm region
We have in-house capabilities for growth of 
crystal, fabrication, design and system insertion.
has excellent in-house capability for crystal growth, design, cutting, 
polishing, AR coating, fabrication, and  system insertion. 
Large crystals of Hg2Cl2 and Hg2Br2
Microgravity Experiment to Grow HgCl Crystals by 
PVT Method
Birefringence Interferograms
for Crystals grown in 1-g
• Birefringence 
Interferograms for crystals 
grown at Raleigh numbers 
(a) 4.55x10-2, (b) 6.25x10-2
and © 9.66x10-2. 
• The birefringence 
Interferograms show 
deteriorating crystal 
quality with increasing 
Rayleigh number
Effect of convective levels on scattering
• Scattering for 
crystals grown at 
Rayleigh numbers 
(a) 4.55x10-2, (b) 
6.25x10-2 and © 
9.66x10-2
• The scattering 
curve 1 is laser 
beam without 
crystal
X-Ray Rocking Curve and Bulk Transparency of 
a Crystal Grown in 1-g
Transport Region for HgCl as the Function of 
Gravity Level 
38Detached growth was observed for the first time in space and in 1-g
Microgravity experiments required several days 
for training of atranauts and space load scientists
Seeded Growth Ampoules and temperature 
profile for microgravity experiment
Morphology of growing crystals
Growth Interface of Crystal A in Space at Different Time
Growth Interface of Crystal on A side
Microgravity Experiment to Grow HgCl Crystals by PVT 
Method 
Microgravity Experiment to Grow HgCl Crystals 
by PVT Method 
Growth 
Interface of 
Crystal B in 
Space at 
Different Time
Growth Interface of B crystal in Space
Microgravity Experiment to Grow HgCl Crystals by PVT 
Method 
Two HgCl Crystals Grown in Space
Experimental and theoretically constructed <110> 
orientation of Hg2Cl2 crystal
•This
Experimental and theoretically constructed <001> 
orientation of Hg2Cl2
This is for HgCl tetragonal crystal
Bulk Transparency of Crystal A and B with and 
without wire mesh
A
B
Rocking Curve for crystal which was not crack 
showed good quality
Microgravity Experiment to Grow HgCl Crystals by PVT 
Method
Summary: Sound ground base experiment is 
essential before  the microgravity experiment
 Bridgman Growth
 Heavy metal halides are excellent Acousto-optic materials
 Direct observations were taken during solidification
 Interface showed toroidal instability in presence of AgBr impurities
 A quantitative correlation was established between convection and
quality of crystals for Bridgman growth (SRD)
 Physical Vapor Transport Growth
 Microgravity PVT experiment was carried out in STS-77 in Spacehab-
04 to grow mercurous chloride crystals
 Two crystals were grown with translation rates of 5 mm/day and 9
mm/day
 Crystals did not follow [110] orientation in Space.
 Crystals grew faster in the center of the ampoule compared to edges.
 Furnace shut down occurred due to another payload purges.
 Crystals quenched with a very fast cooling rate.
 Quality of small crystals ( taken from uncracked portion) were very
much superior than 1-g grown crystals
Materials are key for the performance of 
systems
Climax and anticlimax 
are part of the life
Help me, I will see everything 
what you have seen, but I will 
think in a way you never did, 
and I will patent it and publish 
papers
No one wants to fail.
Everyone wants 
society, love and 
appreciation
Guide me by coaching, I 
will learn from my own 
mistakes
No one wants to fail, everyone needs appreciation and little help to achieve success
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Double-Diffusive Convection During Growth of Halides

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