Report on WP1/M1: Investigation on silicate bonding at low temperatures
E. Elliffe1, S. Reid1, S. Rowan1, J. Hough1
1
Institute of Gravitational Research, University of Glasgow, Glasgow, UK
Coordinators: V. Loriette (CNRS), M. Punturo (INFN)
Introduction
The technique of hydroxide-catalysis, or ’silicate’ bonding was invented, and patented
at Stanford University by D-H Gwo for the purpose of jointing the fused silica pieces
forming the startracking telescope assembly used in the Gravity Probe B space
experiment, successfully launched in April 2004. The purpose of the invention was to
enable optical components to be assembled seamlessly together without the
disadvantages of optical contacting. The bond material had to be transparent in the
visible, vacuum compatible, create no magnetic disturbance, be capable of handling
rapid temperature changes and launch conditions. This technique was then
successfully adapted and adopted for use in the construction of the all-fused-silica test
mass suspensions of the GEO600 interferometric gravitational wave detector, and is
planned for use in the fused silica suspensions of ‘Advanced LIGO’, the planned
upgrade to the US LIGO detector systems.
Bonding technique
Hydroxide catalysis bonding is used to bond solid materials whose surfaces can be
hydrated and dehydrated through hydroxide catalysis, in a reliable manner. The
technique employs aqueous solutions of hydroxides consisting of a hydroxide with or
without some kind of silicate.
Silica surfaces that are stripped of adsorbates and are immersed in water for long
periods of time allow hydration to tend towards a maximum. To increase the
likelihood of a successful bond the silica surfaces are rigorously cleaned thus
maximising the hydrophillicity of the surface. A measure of this surface state can be
made, by checking the contact angle de-ionised water makes with the surface. When
the de-ionised water sheets across the surface to be bonded in a uniform manner,
without any sign of coagulation, then the contact angle is said to be minimised
(approximately zero degrees), and thus the hydrophilicity of the surface is maximised.
This produces polymeric chains of -Si(OH)2-OSi(OH)2-OH groups which can link up
in many different ways to form a 3-D network on the surface. When this process
occurs at a silica-water interface or silica-hydroxide solution interface, it is believed
to produce short protruding chains of -Si(OH)2-O-Si(OH)2-OH. These chains can
lose one or more protons to become silicic acid. -Si(OH)2-O-Si(OH)2-O-. When two
solid surfaces that exhibit this nature are brought into contact and are sufficiently flat
(approximately /4 or better, where  600nm) a joining/merging of these 3-d
networks will occur by the forming of siloxane chains between the two surfaces.
As discussed by Gwo bonding may occur between flat surfaces of a number of
materials if a silicate-like network can be created between the surfaces. The most
obvious of these are silica glass or glass ceramic composites such as ZERODUR or
ULE. The chemistry of hydroxide catalysis bonding can be considered to take place in
three steps, namely, hydration and etching, polymerisation and dehydration.
To allow bonding to take place, an alkaline bonding solution typically sodium or
potassium hydroxide or sodium silicate is placed on one of the surfaces to be bonded
and the other silica surface is then placed gently in contact. The OH− ions in the
bonding solution act as a catalyst and etch the silica surfaces in contact with the
solution: as a result the surfaces liberate silicate ions.
As silicate ions are released into the bonding solution the overall number of active
OH− ions in solution are reduced, the silicate ion disassociates to form Si(OH)4.
These Si(OH)4 molecules can then combine and polymerise to form siloxane chains
and water.
The siloxane chains provide an overall rigidity at the interface and start to form the
bond as the water evaporates or migrates into the bulk of the material. As the
dehydration continues, a 3-d network of tangled siloxane chains rigidly joins the
surfaces.
First samples of silicon bonded
We have initiated the fabrication of test bonds to silicon, initially of fused silica to
silicon such that the bond itself can be easily viewed. This has been successful.
We procured suitable substrates for bonding, characterised the flatness of the surfaces
to ensure they were suitable, cleaned and prepared the surface of the samples for
bonding and bonded the samples using sodium silicate solution as described above.
We performed initial tests to check that this type of bond can withstand cooling to
LN2 temperature – these tests were successful and indicated no apparent change in
the appearance of the bond.
We have procured a further small number of silicon samples for bonding tests which
will continue over the next period.
Fused silica
substrate
Silicon
substrate
Figure 1 Sample of silicon bonded to fused silica using the technique of silicate bonding.