Steve Willner wrote:
In article ,
(Robert Clark) writes:
Note that the glass used for large telescope mirrors is used to
maintain their shape because of its strength and lightness.
Glass is used because it can be polished to a smooth and accurate
finish. It is both weak and heavy compared to other structural
materials. Unfortunately other materials all have worse problems, at
least so far, although metal mirrors have been used in telescopes
from several hundred years ago until now (e.g., SST).
reflectivity and smoothness comes from a thin layer of metal
applied
to the surface.
Reflectivity yes, smoothness no. The metal coating is typically
about 1/1000 of a wavelength thick, far too thin to affect the
smoothness.
A big problem with multi-meter telescopes is the mirror starts to
deform under it's own weight. However, there are several methods
available now to create diamond in large amounts:
Diamond would be a great material if it could be produced in large
sizes and polished to an acceptable shape and finish. I don't expect
either production or polishing will be easy.
Silicon carbide has many of the same advantages as diamond. I
understand SiC mirror blanks have been produced in meter sizes, and
there are claims that people have polished mirrors, but I don't know
offhand of any examples in use. Supposedly a company in Russia was a
source of SiC mirrors, but I don't know which company or what their
capabilities might be.
--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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Thanks for the response. I've seen different values for the Young's
modulus of diamond. The highest I've seen is 1200 gigapascals in the
[111] direction. I'll take this value. BTW, the modulus for silicon
carbide is 420 GPa.
So at 90 GPa for the Zerodur low exansion glass used in optics,
diamond is better by a factor of 13.3. Yan et.al. who demonstated the
high growth rate CVD method for diamond say their spectrospopic
measurements suggest the 50% increase in hardness extends through the
entire diamond, not just the surface. This suggests there should be an
accompanying increase in strength of 50%, to 1800 GPa. I'll take this
value. So this new CVD diamond is 20 times as strong as Zerodur.
This report compares the physical properties of some materials used
for mirrors:
Primary Mirror Substrate Materials for the XLT Telescope:
A comparison of various options including Silicon Carbide
http://www.hia-iha.nrc-cnrc.gc.ca/VL...ts/XLT-SiC.pdf
On page 10 is given a formula for calculating the root-mean square
deflection for a mirror from its own weight, based on size, material,
and number of supports for the mirror. From this formula, we can
conclude it's proportional to Density*(1-Poisson's ratio^2)/Young's
Modulus. Then the deflection for diamond is smaller than Zerodur by a
factor of (3.52/2.52)*(1-.2^2)/(1-.24^2)*(1/20) =3D .071. It's also
smaller than the deflection of a SiC mirror by a factor of .26.
We can also see from the formula that if a mirror is scaled up by a
constant factor k in radius and thickness, then the deflection is
changed by a factor of k^2. Then since .2664^2 =3D.071, we can get the
same level of stability from a diamond mirror as a Zerodur one that is
..2664 times as big. So a diamond mirror 8*.2664 =3D 30 meters wide would
have comparable stability against deformation to a current Zerodur
mirror 8 meters wide.
A diamond mirror this size would be quite heavy. Note though that
assuming diamond material can be made in arbitrarily large sizes, then
for the support we could also use diamond pillars to support the
mirror. This would be several times stronger than steel for the weight.
It would also have the advantage that the thermal expansion for the
support structure would match that of the mirror.
In regards to increasing the size of the CVD grown diamonds, this
review article suggests the growth rate scales linearly with energy of
the microwaves used:
CVD Diamond - a new Technology for the Future.
"One of the great challenges facing researchers in CVD diamond
technology is to increase the growth rates to economically viable
rates, (hundreds of =B5m/h), or even mm/hr) without compromising film
quality. Progress is being made using microwave deposition reactors,
since the deposition rate has been found to scale approximately
linearly with applied microwave power. Currently, the typical power
rating for a microwave reactor is ~5 kW, but the next generation of
such reactors have power ratings up to 50-80 kW. This gives a much more
realistic deposition rate for the diamond, but for a much greater cost,
of course."
http://www.me.berkeley.edu/diamond/s...iew/review.htm
The paper by Yan et.al. discusses using a 6 kW microwave oven for
their CVD process:
Very high growth rate chemical vapor deposition of single-crystal
diamond.
PNAS | October 1, 2002 | vol. 99 | no. 20 | 12523-12525
http://www.pnas.org/cgi/content/full/99/20/12523
Using this they were able to get up to 150 micron/hour growth rates.
If the Yan et.al. process also scales linearly as other microwave CVD
methods, then a 6 megawatt microwave reactor would give a 150
millimeter/hour growth rate. So production of a 30 meter mirror would
require 30,000/150 =3D 200 hours, less than 9 days.
Another method would be to use several microwave ovens of the same 6kW
size used by Yan et.al. simultaneously, each working on its own seed
diamond. If we used a hundred of these we could get an equivalent total
size of a 30 meter mirror in 90 days.
As each segment approached the desired size, we would want them to
connect to form a single mirror. We could do this by sending a plasma
gas between two formed segments to get a single crystal diamond, just
as the original process forms a single crystal diamond on a single
surface. We would have to carefully match up the crystalline directions
in the separate segments so that the plasma could form a single crystal
consistently on both surfaces. We might insure this by cutting the
separate seeds from a single crystal.
The CVD method also makes it easier to form the shape of the final
mirror. We could cut the diamond seed(s) into the desired parabolic
shape and the CVD deposition would follow this shape. To get the fine
smoothing of the mirror surface, we could control the deposition of the
plasma using electrostatic or magnetic fields, as used for example with
Penning traps.
Another method might be to use laser deposition to get the final
mirror surface. This method produces polycrystalline diamond rather
than single crystal diamond, so it is not strong as the Yan et.al. CVD
method, but it allows finer control by directing the laser. However,
since this would be used to only deposit a thin layer on the top it
would not have to support much weight:
BRILLIANT DISCOVERIES
DIAMONDS ARE A PART'S BEST FRIEND
"A diamond coating breakthrough
"A major breakthrough in diamond deposition technology occurred when
Pravin Mistry, a metallurgist, was doing independent materials research
and consulting to industries requiring better tooling for metal forming
and extrusion. He was working towards fabricating hard materials using
lasers to synthesize ceramics and metal-matrix composites (MMC) on
aluminum extrusion dies to improve their performance and longevity. In
a fortunate misstep during laser synthesis of titanium diboride, Mistry
switched carbon dioxide for nitrogen and produced a coating speckled
with some black particulate inclusions.
"Analysis of the coating's surface indicated the presence of
poly-crystalline diamond. Retracing the steps of his experiment, Mistry
conceived a radical method for synthesizing polycrystalline diamond
films. The QQC Diamond coating process uses the carbon dioxide from the
atmosphere as the carbon source and subjects it to multiplexed lasers
to produce diamond film that can be deposited onto almost any
material."
http://www.advancedmanufacturing.com...ploringamt.htm
Bob Clark