Work package 1, task 1: M1 – Advanced materials for mirror substrates
Task aims in the 3rd year
 Investigation on the doping effect on Si.
 Investigation of silicate bonding at low T.
Major achievements in the third 12 months
Si doping:
Influence on thermoelastic properties
We are using bulk cylinders of silicon as samples on which to study the intrinsic dissipation of
silicon, at resonant frequencies in the kHz region where thermoelastic damping is negligible and
thin silicon cantilevers, with resonant frequencies in the tens of Hz to ~1 kHz to study levels of
thermo-elastic damping, in the frequency range of interest for gravitational wave detection.
The bulk samples have been fabricated from both commercial boron-doped silicon and also
nominally undoped material in an attempt to identify any differences in behaviour associated with
the different doping levels.
However so far we believe that any differences we see in the samples are a result of the cut of the
sample influencing the measured Q, possibly through the effect of the crystal orientation on mode
shape and thus coupling to the support structure.
To further investigate this possibility we have procured samples of the same doping but of
different aspect ratios and cuts. Currently our measured loss factors are consistent with our
postulate that we are not yet measuring dopant-related dissipation, but dissipation perhaps due to
the support structure used.
Previous work by Numata et al showed that for some subset of modes of a sample a support,
which contacts the sample at positions of minimum or zero displacement, can give low support
losses. We have thus constructed a prototype ‘nodal support’ system to attempt to reduce
suspension losses for these modes, and allow dopant related properties to be studied.
Colleagues in Jena are also studying the loss factors of bulk silicon samples and plans to
exchange samples for joint study are currently under discussion.
Once support losses have been reduced further we will revisit the study of the mechanical
dissipation of doped versus undoped silicon samples and then use the results of that study to
inform further experiments to study silicon samples with different levels of doping.
Delays and possible alteration of the planning
Due to the exisitence of excess loss associated with the support, we plan to continue these
investigations over the next period.
Influence on optical properties
Our previous absorption bench was not able to test samples having resistivities higher than
2kohm.cm. We have built of a new bench based on a new modulation technique. The bench was
finalized in January. The increase of sensitivity is close to 3, which allowed us to test all of our
samples. We have shown that down to 4kohm.cm absorption by free carriers is one of the still the
main absorption sources.
We have not found any manufacturer that could control or specify the amount of impurities
present in quasi-intrinsic silicon (>5kohm.cm) unless some R&D is performed, and no
manufacturer shows any interest in carrying such R&D activity. We will have to rely on high
resistivity silicon for controlling the absorption level.
Delays and possible alteration of the planning
We are confident that although he bench will be operational late, the results will be obtained
before the end of year 3. However it is not at all evident that we will find commercially available
silicon having the right dopant concentration (rho>4kohm.cm) with controlled impurity content
Silicate bonding
Pairs of silicon disks have been successfully bonded in Glasgow using different volumes of
sodium silicate bonding solution (1 part commercial sodium silicate solution to 6 parts water,
using volumes in the range 0.4ul/cm2 to 0.1ul/cm2).
A complete set of these samples has been sent to Florence for thermal conductivity
measurements, having been given 2-3 months for curing.
In addition to thermal conductivity tests, mechanical strength measurements are planned both at
room and low temperature (see the M5 report).
Initial mechanical testing at room temperature shows that a 5cm2 silicon-silicon bond is capable
of supporting a 40Kg load over a period of two weeks with no observed distortion or damage (a
shear force of ~100 N/cm2, 1e6 Pascals).
Figure 1. Preparation of silicon samples to be bonded: the required surface flatness ≤ lambda/10,
the surfaces were thoroughly cleaned from contaminants prior to bonding, and oxidised prior to
silicate bonding.
cleaned silicon samples
placed in furnace at 1000°C
after ~1hr, 50 to 100nm oxide growth
Considerable studies have been carried out of the dependence of bond settling time on the
temperature and pH of the bonding solutions used and a paper on this work is currently in
preparation. (Reid et al, 2006)
Delays and possible alteration of the planning
We plan to continue these investigations over the next period.
Work package 1, task 2: M2 - Advanced materials and techniques for resonant
detectors
Task aims in the 3rd year
 Measurement of mechanical loss properties at low temperature in advanced materials
for resonant detectors.
 Investigation of the fabrication processes of silicate bonding for SiC, and electron
beam welding, explosion welding and cold welding for metals.
Major achievements in the second 12 months
Materials:
Many cryogenic tests on different SiC samples have been performed. The best achieved
low temperature (2K) quality factor is about 4e5. This value is far from the Dual detector
requirements, however there is a reasonable chance to get a higher value during the next
ultracryogenic run.
A low loss nodal suspension for silicon disks has been developed. Using this suspension
we measured the mechanical quality factor of silicon disks giving at low temperature Qvalues up to 5e7, dominated by the thermoelastic losses. The displacement of the disk has
been measured using both a new developed capacitive transducer optimized for dielectric
samples and an optical lever transducer.
Different welding technologies as Electron Beam, Diffusion Welding, etc have been
investigated for bonding two hollow hemisphere of CuAl.
The best result, so far, was obtained with the method of furnace brazing. The so
fabricated hollow sphere showed an elastic behaviour as expected from the elastic theory
of a continuum body and a low temperature quality factor reduced only a factor 40% with
respect measured value on a bulk sphere.
Transducers:
Many high sensitive capacitive transducers for the MiniGrail detector have been realised
and tested at room temperature. The transducers are assembled with the innovative
method of gluing together all their components. Thanks to the large area, the transducer
sensitivity is enhanced compared to the previous generation. Three different double
SQUID amplifiers for the new capacitive transducer are under test.
The achieved breakdown field on small Aluminium samples is about of 50 MV/m. Test
on large area sample has begun with encouraging results.
Finally in order to reduce the electrical thermal noise which severely affects the acoustic
detector sensitivity, the loss angle of sapphire in the kHz region has been measured as a
function of temperature. Down to 400 mK the estimated value of the loss angle is slightly
higher than the one of PTFE, the commonly used dielectric material for the capacitive
transducer and scales as T1/2.
Delays and possible alteration of the planning
None expected
Work package 1, task 3: M3 - Investigation of superconducting materials
Task aim in the 3rd year
The aim of this task is to build cavities using the technique of niobium sputtering on a
substrate having high thermal conductivity and low intrinsic mechanical dissipations.
Major achievements in the third 12 months
Measurement of mechanical modes (frequency and dissipation) at room and Cryogenic
temperatures performed. The results have been used to update the design for the
prototype construction.
The Niobium sputtering has been optimized and the test of a Niobium sputtered cavity
welded an Electromagnetic Quality factor of 1010 at a stored energy of 10 Joules. These
values are better than the minimal specifications required for the first detector prototype.
Prototype Cavity built without any modification with respect to the design and the
construction drawing. Prototype will be delivered around middle October. The integration
in the Cryogenic Test Facility will start immediately after. First data available before the
end of the year
The cavity was tested at 1.8K in January 2007 showing a Quality factor of 2x1010 at a
stored energy of 12 Joules, exceeding (also for the full niobium prototype) the design
goal.
Delays and possible alteration of the planning
None expected
Work package 1, task 4: M4 - Development of low loss dielectric coatings for
advanced detectors
Task aims in the 3rd year
 Measurement of mechanical losses, optical losses and index of refraction performed
at room and low temperatures.
 First results from diffractive coatings
Major achievements in the third 12 months
Temperature dependence of the mechanical dissipation in single layer of Ta2O5
doped with TiO2 and Co carried out at the Universities of Glasgow and Perugia
The measured loss factors at a range of temperatures for the fourth and fifth bending
modes of a coated and uncoated pair of silicon cantilevers were studied in Glasgow. The
coating comprised of a single-layer of ion-beam sputtered Tantala (Ta2O5) doped with
14.5% Titania (TiO2). The cantilevers were fabricated from n-type Phosphorous doped
silicon with a thickness of 52m and resistivity 5-10 cm. Results of both modes
repeatably show an increase in the mechanical loss of doped tantala coatings as the
temperature decreases. Understanding the underlying mechanisms responsible for the
observed increase in loss and investigating methods by which this may be reduced are of
significant importance in regards to the sensitivity of future detectors. Therefore, to
investigate this possible dissipation peak in greater detail, the cryostat facility in Glasgow
has now been commissioned to operate down to 4K. Initial measurements of a similar
blank silicon cantilever show the measured losses reaching levels of around 1×10-7 at 4K.
In Perugia, loss measurements of coated and uncoated silica slabs have been carried out
between 300K and 4K. Studied here were a Titania (TiO2) doped Tantala (Ta2O5)
coating and a Cobalt (Co) doped Tantala coating, both on fused silica substrates. The
calculated loss associated with the coating layers of each of these coatings are shown in
Figure 1. In order to determine the loss of the coating material itself, the ratio of elastic
energy stored between the coating layer and the bulk substrate had to be evaluated. Finite
Element analysis was used to evaluate these energy ratios. Analytical calculations were
also used in Glasgow and found to agree within a few percent with the FE model.
c
d
b
a
Fig1: calculated coating loss (a) & (b) of a titania doped tantala coated silicon cantilever
at 777Hz and 1280Hz respectively measured in Glasgow, (c) of a titania doped tantala
coated silica slab and (d) of a cobalt doped tantala coated silica slab measured in Perugia.
Fabrication of coatings at LMA in Lyon.
Studies on the mechanical loss and the optical absorption in dielectric coatings for highreflectivity have been carried out by LMA. A summary of the various fabricated and
studied coatings are shown in Table 1.
Coating material
Ta2O5
Ta2O5 : W
Ta2O5 : W+Ti
Ta2O5 : Co
ZrO2
ZrO2 : Ti
ZrO2 : W
Absorption (ppm) Mechanical losses
Ta O  3.0 104
Ta O :W  7.5 104 
0.5
2.45
1.65
5000
11
37
10
2
5
2 5
Ta2O5 :W Ti  3.2 104
Ta2O5 :Co  1.1103 
 ZrO2  2.3 104
 ZrO2 :Ti  6.8 104
 ZrO2 :W  2.8 104
Table 1: optical absorption and mechanical loss of various coating materials
(coating thickness 500nm, * indicates before annealing)
Currently the only high index material capable of fulfilling both the mechanical and
optical requirements for future interferometers is titania (TiO2) doped tantala.(Ta2O5).
Studies will continue over the next period to investigate the dissipation mechanisms
within the doped tantala coatings and investigate ways in which they can be reduced.
Effort will also be given to reducing the optical losses of other coating materials down to
a suitable level in addition to looking for new potential materials.
First results of diffractive coatings (Universities of Glasgow and Jena)
Initial measurements to study the effect of diffractive gratings on the mechanical loss of
mirror substrates have been carried out. Progress towards measuring these samples with
both the diffraction gratings and an optical coating has been hindered when both samples
were damaged by the different coating vendors during coating. New silica disks with
diffraction gratings have been tested in Glasgow and have been sent to LMA for coating.
However, in spite of the investigation being incomplete at this stage, initial measurements
suggest that the diffractive coatings themselves contribute negligible excess mechanical
loss to the substrates.
Measured loss 
Jena
Glasgow
blank SiO2 disk
SiO2 disk with grating
-6
10
-7
10
0
4000
8000
12000
16000
20000
Frequency (Hz)
Fig 8: Comparison of measured losses of blank
and diffraction grating fused silica disks
(75×1.7mm, 50nm etched grating)
Fig 9: Comparison of measured losses of blank
and diffraction grating quartz substrates
(75×12mm c-axis substrate, 2m period, 115nm grating)
Delays and possible alteration of the planning
Measurements of optically coated diffractive gratings will be delayed around 6 months
for the reasons detailed. No other delays are expected.
Work package 1, task 5: M5 - Innovative materials for suspension in advanced
detectors
Task aims in the 3rd year
 Status of bulk loss measurements on silicon
 Preparation of silicate bonded samples for testing
 Setup of a cryogenic facility in Firenze for thermal expansion and conductivity
Major achievements in the third 12 months
Bulk loss measurements of silicon
Current loss measurements of bulk silicon substrates predict very low levels of thermal
noise for a third generation detector operating with silicon optics. Therefore silicon
substrates do not currently represent a limit to the sensitivity of such interferometers.
Doping and surface effects are likewise regarded as not as immediately critical as they
were considered at the beginning of the project. Bulk silicon studies will continue in
Glasgow and Jena alongside studies of the effect of dielectric coatings on bulk substrates
at room and low temperature (for coatings R&D status - see M4 report). So far we
believe that any differences we see in the samples are a result of the cut of the sample
influencing the measured Q, possibly through the effect of the crystal orientation on
mode shape and thus coupling to the support structure. Plans are underway for
exchanging samples between Glasgow and Jena for comparative measurements. In
addition to this, a nodal support is being commissioned in Glasgow which, for a subset of
resonant modes, contacts the sample at positions of minimum or zero displacement.
Silicate bonding characterisation
As detailed previously, pairs of silicon disks were successfully bonded in Glasgow using
different volumes of sodium silicate bonding solution (1 part commercial sodium silicate
solution to 6 parts water, using volumes in the range 0.4ul/cm2 to 0.1ul/cm2). This initial
set of samples is being studied for their thermal conductivity across the bond region in
the cryogenic facility at Florence.
In addition to thermal conductivity studies, the construction of a thermal expansion
measurement facility is in progress in the Florence lab and employs a Michelson
interferometer with a controlled reference cavity.
Cryogenic facility in Firenze
Firstly, a reference resonant cavity has been built, for stabilizing the laser frequency. A
Zerodur hollow cylinder has been placed on a V-shaped steel support, and fixed by a
collar. The Zerodur cylinder, which has a very low expansion coefficient, provides the
length reference; the cavity mirrors are pressed against its ends by springs. The
piezoelectric actuator, which allows the variation of the cavity length, is inserted between
one mirror and the cylinder.
Then, the cavity arrangement has been placed inside a vacuum tank. The tank is
connected in parallel with the vacuum system of the measurement cryostat. Also a
vacuum gauge and the viewports for injecting and seeking the laser are mounted.
The reference length stability depends on temperature fluctuations; in order to achieve
the design sensitivity (of the order of percent for L/L~10-8) these fluctuations should
stay under 10 mK. Thus, a temperature control on the cavity has been realized. The
vacuum tank has been wrapped with a copper coil to be employed as heater, and a
PT1000 temperature sensor has been stuck on its surface, while the whole cavity has been
insulated from the environment in a polyurethane box. Then, a PI controller has been
projected, built and tested, resulting in temperature stabilization at 316 K with residual
fluctuations of few mK.
Once the reference cavity was ready for the laser to be injected, the optics for the
injection have been designed and mounted on the bench. The optics consists of a Faraday
isolator, an electro-optical modulator (for the Pound-Drever locking technique) and a
telescope for matching the beam to the cavity. Also a dog legs system has been mounted
for aligning the beam, and a polarizing beam splitter which convoys the beam on the
read-out photodiode.
At the moment, the aligning of the beam is in progress; the next step will be the locking
of the reference cavity.
A facility for measuring thermal conductivity in small samples (which can be hosted
within our cryostat, that is <30 cm) has been built and tested. It gave, for a high purity
silicon sample 8 cm long, 5 mm thick, a curve k(T) which goes from room temperature to
about 40 K and is in good agreement with literature data in the range 100-300 K where
silicon specimens are all expected to behave in the same way.
The measuring system is hosted inside a LN-LHe cryostat, also connected to a vacuum
pumping stage. The residual pressure, checked by means of vacuum gauges, can reach
values of about 10-5 mbar.
The conductivity is estimated by measuring the stationary temperature gradient along
the sample, when a known heat flux, provided by a coil heater, is set across it.
The heat sink has been realized with a massive conductive clamp, which was insulated
from the cold plate of the cryostat with a PVC layer. Such layer makes the temperature
vary slowly, and that allowed to perform measurements comfortably at each temperature.
Temperature gradient is acquired by using two PT1000 sensors connected in a bridge
configuration. The bridge is fed with an AC signal, and the output is demodulated via a
lock-in amplifier.
Currently, we are performing a measurement of thermal conductivity on silicon disks (6
mm thick, 1” diameter), bonded by Glasgow using the silicate bonding technique (see the
M1 report). Such measurement requires an upgrade and a modification of the setup to
take in account the different geometry of the samples (disks of 1” diameter and 6mm
thickness). Due to the large surface compared with the small thickness, a large heat flux
is needed for having a measurable temperature gap across the Si disk and the silicate
bonding. The samples are now clamped between the coil heater and a Cu sink, directly in
thermal contact with the cold plate of the cryostat. The large heat capacity of the system
allows to perform measurements during the very slow heating of the cryostat from
cryogenic up to room temperature. Three ring-shaped sensors, equipped with PT1000, are
placed along the lateral surface of the cylinders to monitor at the same time the
conductivity of Si and of the silicate bonding.
Presently the main difficulties faced are connected with the thermal contacts between the
samples and the heater or the sink: if the contacts are not uniform over the whole surface
the resulting non homogeneous heat flux compromises the measurements. The solution to
this problem seems to be the use of a thin layer of Indium.
Delays and possible alteration of the planning
We plan to continue the investigations on bulk loss measurements on silicon and silicate
bonding characterization over the next period.
Work package 1, task 6: M6 - Study of thermoelastic effects caused by absorption of
cosmic rays
Task aims in the 3rd year
 Low temperature measurements on an Aluminium bar
Authors
Major achievements the third 12 months
The investigation of the thermo acoustic effect induced by the interaction of ionizing
particles in resonant detectors is approaching the final measurement.
During last year the cryogenic system has been upgraded with the installation of the new
dilution refrigerator needed for the measurement Al5056 in superconducting state. The
system, delivered at the beginning of 2006, required the machining of several parts in
order to adapt the dilution refrigerator to our cryostat, so that the hardware setup has been
completed only in August.
The first cryogenic test, performed in September, showed a problem in a bellows of the
dilution refrigerator, compromising the performance of the apparatus. A second
cryogenic run is scheduled in the begging of November, followed by the Al5056
measurement in superconducting state soon after.
In the main time some analysis is done on the Niobium bar data acquired in 2005. Data
“unexpectly” show that the amplitude of the signal is depressed below the transition
temperature: this behavior is not expected from the standard thermo-acoustic model,
predicting that superconductivity is locally broken by the increase of temperature due to
the energy lost by particles and normal state thermal expansion coefficient and specific
heat should be used. The results seem to be in accordance with a model where two terms
should be added in the signal amplitude: a phase transition between superconducting and
normal state; the standard thermo acoustic model along the particle track. The
collaboration successfully published the data analysis of the results obtained with the
niobium bar.
Delays and possible alteration of the planning
None expected.