Can diamond now be used for telescope mirrors?

Note that the glass used for large telescope mirrors is used to maintain their shape because of its strength and lightness. The
reflectivity and smoothness comes from a thin layer of metal applied to the surface. 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:
Large Diamonds Made From Gas Are The Hardest Yet. http://www.sciencedaily.com/releases/2004/02/040226070311.htm
Bob Clark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com (Robert Clark) writes:

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 yes, smoothness no. The metal coating is typically about 1/1000 of a wavelength thick, far too thin to affect the smoothness.

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 snipped-for-privacy@cfa.harvard.edu
Cambridge, MA 02138 USA
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 8 Dec 2004 16:45:28 -0400, snipped-for-privacy@cfa.harvard.edu (Steve Willner) wrote, in part:

The Europeans are orbiting a space telescope with one.
John Savard http://home.ecn.ab.ca/~jsavard/index.html
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 09 Dec 2004 01:27:48 GMT, snipped-for-privacy@excxn.aNOSPAMb.cdn.invalid (John Savard) wrote, in part:

I should have been more clear: preparing to orbit a space telescope with one, the Herschel space telescope.
John Savard http://home.ecn.ab.ca/~jsavard/index.html
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
(John Savard) writes:

SiC has also been used for x-ray mirrors.
Mati Meron | "When you argue with a fool, snipped-for-privacy@cars.uchicago.edu | chances are he is doing just the same"
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@excxn.aNOSPAMb.cdn.invalid (John Savard) writes:

Thanks. Details at http://sci.esa.int/science-e/www/object/index.cfm?fobjectid4705
As far as I can tell, the shortest wavelength Herschel will observe is 60 microns. This means the mirror polishing can be about ten times worse than would be necessary for visible observations. That doesn't mean better polishing can't be done, of course, only that it is unlikely to be demonstrated by this telescope.
--
Steve Willner Phone 617-495-7123 snipped-for-privacy@cfa.harvard.edu
Cambridge, MA 02138 USA
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Steve Willner wrote:

applied
snipped-for-privacy@cfa.harvard.edu

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/VLOT/Reports/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) = .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 =.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 = 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 m/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/submissions/diam_review/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 = 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/November02/exploringamt.htm Bob Clark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Robert Clark wrote:

6kW
total
Correction. If several 6 kw ovens were used to make separate segments it would take more ovens than this or a much longer time. If 900 ovens were used, you would you get 900 segments each 1 m wide. At 150 microns/hour this would take 1000 mm/.150 mm/hr = 6667 hours, or 278 days.
Bob Clark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Robert Clark wrote:

Oh, nuts. NOW you tell me. After I bought the ovens! :-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Don't want to get involved, but your extrapolation of 1/2.664 = 3.75 to get a 30 m mirror from an 8m is incorrect. The drooping is proportional to the square of the diameter. The stiffness is calculated using moment of inertia mr^2 etc. I believe you can only go by square root = 1.93 to get a large mirror 15.5 m. Of course, there's the saving in ovens :). John Polasek If you have something to say, write an equation. If you have nothing to say, write an essay
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'm a little confused by what you quoted above. That was only used to say if you have 30m by 30m mirror (taken square for simplicity.) Then this could be made up of 900 segments each 1 meter wide. I assume you were actually referring to this earlier passage:
"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 =.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 = 30 meters wide would have comparable stability against deformation to a current Zerodur mirror 8 meters wide."
Several references give the deflection amount according to the material and size. Here's one:
Mirror Structural Design. http://astron.berkeley.edu/~jrg/MirrorStructure/node1.html
It shows the deflection is proportional to (diameter)^4/(thickness)^2. So if the dimensions are increased uniformly by a factor k, the deflection goes up by k^2. By replacing low expansion glass by diamond you want to see how much you can increase the size when taking into account diamonds greater strength ratios so that you don't incur greater deformation. You therefore take the square-root of the increase in material strength to see by what factor you can uniformly scale up the size of the mirror.
Bob Clark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 14 Dec 2004 09:52:31 -0800, "Robert Clark"

According to Marks' Handbook, the deflection of a circular plate of stiffness Y supported at the edge and subject to a uniform pressure P is     Defl = (r^2/t^2)*P/Y But the gravity pressure goes up as the weight, g* rho*(area*t)/area rho*t so    Defl = (r^2/t^2)*rho*t/Y = (rho/Y)*(r^2/t) From this if you double the radius the stiffness must go up by 4 and you have to take into account the density rho of the material. Y/rho is a sort of specific stiffness, being c^2 for a particular material (m/l).
But you don't have to have the same deflection, you could allow yourself a double deflection for doubling the radius, since     %error = defl/radius. John Polasek
If you have something to say, write an equation. If you have nothing to say, write an essay
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John C. Polasek wrote:

to
Then
is
would
(diameter)^4/(thickness)^2.
diamond
increase
up
Can you give me the bibl. ref. for that handbook you mentioned? Every reference I've seen gives the deflection for a telescope mirror as proportional to (diameter)^4/(thickness)^2. But reading that passage again I'm chagrined to see I wrote "So a diamond mirror 8*.2664 = 30 meters wide would have comparable stability against deformation to a current Zerodur mirror 8 meters wide." Ack! Obviously if I'm saying the diamond mirror will be bigger I should *divide* by .2664! That equation should be 8/.2664 = 30. Bob Clark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 14 Dec 2004 18:26:51 -0800, "Robert Clark"

It's Mechanical Engineer's Handbook, Lionel S. Marks, Fourth Edition, McGraw-Hill pg. 477 Fig. 43. It's an old one. I also made a mistake. He has two formulas, the first being for S stress that I quoted you and next to it, the one for deflection being proportional to r^4:     defl = r^4*P/t^3*Y     P = g*rho*(area*t)/area = g*rho*t so     defl = g(rho/Y)*r^4/t^2 So in addition to picking out stiffness Y, you have to compare it to its density rho. If they each increase by n%, you haven't gained anything. Steel has (about) 28e6/7 and aluminum 9e6/3 so it's about 4:3 advantage for steel. Sorry for giving you that scare. But if it's still diamonds you're talking about, cash in the diamonds, buy a new Buick and retire! :) John Polasek
If you have something to say, write an equation. If you have nothing to say, write an essay
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.