S.Reid (Glasgow)

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Transcript S.Reid (Glasgow)

Probing the atomic structure of mirror coatings
using transmission electron microscopy
Stuart Reid, Riccardo Bassiri1 , Konstantin B. Borisenko2 , David J. H.
Cockayne2 , Keith Evans1, James Hough1, Ian MacLaren1, Iain Martin1,
Sheila Rowan1
SUPA, University of Glasgow, 2 Department of Materials, University of Oxford
GWADW, Kyoto, Japan – May 2010
Coating development critical for the future
1st generation
Strain [Hz
2nd generation
3rd generation
Frequency [Hz]
Coating thermal
• The test mass mirror coatings are estimated to be a significant source of
thermal noise in future ground-based GW detectors
• Thermal noise is proportional to the mechanical loss
(internal friction) of the material
GEO600 test-mass
• Considerable research is being conducted into understanding the material
properties of these coatings (see previous and following talks)
– The focus here is on the atomic structure and how this affects the
material properties - and in particular the mechanical loss
What is causing the mechanical loss on
an atomic level?
Transmission electron microscopy
• Useful for probing atomic structure and chemistry
Direct beam
Transmission Electron Microscope
Tecnai T-20
Interactions of electron beam
with sample
• Allows us to characterise atomic structure
– Imaging
– Diffraction
– Spectroscopy
Transmission electron microscopy
Initial interesting results:
Image of multilayer coating,
(bright- silica, dark - tantala)
(from Ta2O5 samples heat-treated at a range of temperatures)
Amorphous diffraction pattern
of 300oC tantala
Crystalline diffraction pattern
of 800oC tantala
• Compare TEM results to mechanical loss
• The 800oC sample has high loss peak at 8090K probably due to crystallisation
• To probe the properties of the amorphous
samples we need Reduced Radial Density
Mechanical loss measurements for heat treated tantala (see previous talk by Matt Abernathy
Reduced radial density functions
Silica and tantala are amorphous materials
– They do not have long range order
– They do have short range order
We can probe this short range order with reduced density functions
– RDFs give a statistical representation of where atoms sit with regards to a central
Tantala diffraction pattern
Intensity profile
Reduced density function
• The reduced density function is a Fourier transform of the information
gained from the intensity profile [D. J. H. Cockayne, Annu.Rev.Mater.Res, 37:159-87, (2007)]
Reduced density functions
• Three Ta2O5 coatings were measured
– Each one was heat-treated at a different temperature (300, 400 & 600oC)
RDFs of heat-treated Ta2O5
– RDFs show differences in local atomic structure as heat treatment temperature rises
– From comparison to the structure of crystalline Ta2O5 we can deduce that the first peak
arises from Ta - O bonds and second peak from Ta - Ta bonds
– Both first and second peaks become more defined and difference in heights between
them decrease as heat treatment temperature rises implying that:
– Material is becoming more ordered
– There is an increase in Ta - Ta bonding
Modelling the atomic structure
– If we accurately interpret the RDF
– What do the peaks mean?
– What bond types correspond to each peak?
We can then:
– Investigate the atomic structure
Reduced density function
– Average distances between atoms
– Co-ordination numbers
– Bond types
– Bond angles
– Probe the material properties
– Relationship to mechanical loss
– Optical properties?
– Other material properties?
Energy optimised Ta2O5 atomic model
(blue - Ta, red - O)
Modelling the atomic structure
Reverse Monte Carlo refinements comparing model to experimental RDFs
RMC refinement
Energy optimisation
Constrains the model
Ensure atoms are sitting in
a physically stable position
Gives a greater degree of
Do experimental and
theoretical RDFs agree?
Initial constraints from the
crystalline structure of Ta2O5
– Atom types
Final structure
– Bond lengths
– Bond types
– Bond distributions
Modelling the atomic structure
RDF comparison before RMC (using initial boundary conditions model)
Refinement process
RDF comparison after RMC (using RMC + energy optimised model)
Modelling the atomic structure
• Reverse Monte Carlo modelling was carried out on the 400oC heat
treated tantala coating
– Results from this model show an
average Ta to Ta bond length of 3.19Å
and Ta to O bond length of 1.99Å
– Co-ordination number for Ta = 6.53,
Crystalline model of Ta2O5
(Aleshina et al., Cryst. Rep. 47, 2002)
– Ring structure of Ta and O bonds
remains partially intact from the
crystalline phase
Energy optimised Ta2O5 atomic model
(Blue - Ta ,Red - O)
Modelling the atomic structure
• Partial RDF data:
– Allows a greater understanding
of the relative distances from
one atom to another
– Shows exactly what the peaks in
the initial RDF mean
– Initial assumptions on comparing
the peaks to the crystal phase
are accurate
RDFs of heat-treated Ta2O5
– Ta - O bonds dominate first
– Ta - Ta bonds dominate second
Partial RDF of 400oC model
Modelling the atomic structure
• Bond types and angles:
– Atomic modelling makes
understanding bond structures
in the sample easier
– Bond types
– Bond angle distributions
Bond type distribution
Bond angle type distribution
• Provides an excellent way to
compare the changes in the
atomic structure
Bond angle distribution of 400oC model
Future work
• Near future
– Compare results from each of
the three heat-treated atomic
– Start modelling Ti doped and
low water Ta2O5 samples
(similar process)
• Future
– Investigate ways of getting
material properties from
– X-ray scattering measurements
for single element ‘RDF’
analysis of Ti doped samples
RDFs of heat-treated Ta2O5
Ti doped tantala RDFs
What is causing the mechanical loss on an atomic level?
• Significant progress towards answering this question
Now have well developed techniques in order to probe the atomic structure
• For Ta2O5 coatings heat-treated to 300, 400, 600 and 800oC:
Samples heat-treated to 300, 400, 600oC are amorphous, the 800oC sample has
crystallised possibly causing the high mechanical loss peak at low temperatures
Preliminary results from the 300, 400 and 600oC show an increase in local ordering and
number of Ta - Ta bonds as heat treatment temperature increases
Atomic modelling provides an accurate way to fully understand the RDF and investigate
bond types and distributions
• Combining microscopic techniques together with mechanical loss
measurements will allow us to gain a better understanding of how these
mirror coatings perform and help produce low mechanical loss coatings
• The same techniques will be applied to other mirror coatings that have
varying material properties